risc-v/gd32vw55x: fix I2C master and add gd32vw553k-start sht3x example

The GD32VW55x I2C master (the STM32-style "v2" IP) never completed a real
transfer. Two bugs:

1. Wrong kernel clock. The protocol state machine is clocked by the I2C
   kernel clock selected in RCU_CFG1.I2C0SEL, not by the APB1 bus clock that
   only feeds the register interface. The driver left it at the APB1 default
   and computed TIMING for PCLK1 (~80 MHz); the resulting prescaled period is
   so short that the SDADEL/SCLDEL setup and hold times fall below the
   analog-filter minimum of the IP, so the master latches START but never
   drives SCL (STAT stuck with BUSY set). Route I2C0 to IRC16M (16 MHz) and
   use it as clk_freq, and never let the prescaler drop below the value that
   keeps the prescaled clock at/under 4 MHz (250 ns) so SDADEL/SCLDEL stay in
   spec. This matches the vendor BSP (IRC16M, PSC=3).

2. Transfer timeout was zero. CONFIG_GD32VW55X_I2C_TIMEOTICKS has a Kconfig
   default of 0 ("override when non-zero"), but the driver derived the timeout
   with a plain #ifndef, which never triggers because the symbol is always
   defined. The polled wait loop therefore ran a single iteration and gave
   up. Address probes (NACK on the first pass) still worked and masked it;
   only multi-byte transfers exercised the loop. Honour the "0 means derive
   from seconds/milliseconds" contract.

Also make the interrupt wait immune to the ISR completing between startmsg()
and the wait (do not clobber a posted DONE), and drop a redundant cast.

To exercise this on hardware, add an "sht3x" configuration to the
gd32vw553k-start: the nsh base plus I2C0, the i2ctool and the Sensirion SHT3x
temperature/humidity driver, registered as /dev/i2c0 and /dev/temp0. I2C0 is
routed to PA2 (SCL) / PA3 (SDA) on AF4, the pins broken out on the J1 header
(datasheet Table 2-5); when SPI is enabled it claims PA2, so I2C0 falls back
to PB0/PB1, whose SDA pin is not broken out (the periph case).

Signed-off-by: Jorge Guzman <jorge.gzm@gmail.com>

Documentation/platforms/risc-v/gd32vw55x/boards/gd32vw553k-start/index.rst

@@ -194,6 +194,50 @@ capture, both watchdogs, progmem, TRNG and CRC), registered as ``/dev/i2c0``,
 ``/dev/spi0``, ``/dev/adc0``, ``/dev/pwm0``, ``/dev/capture0``,
 ``/dev/watchdog0`` and ``/dev/watchdog1``.  No radio.
 
+sht3x
+-----
+
+NSH plus I2C0, the i2ctool and the Sensirion SHT3x temperature/humidity
+driver -- a small example to validate the I2C master port against a real
+device. Because SPI is *off* here, I2C0 is routed to the J1 header on
+**PA2 (SCL)** and **PA3 (SDA)** (adjacent pins on the second row of J1, both
+AF4); with SPI on (the ``periph`` case) it falls back to PB0/PB1, whose SDA
+pin is not broken out. See the pin-header tables above.
+
+Wire the sensor to J1: SCL -> PA2, SDA -> PA3, VCC -> +3V3 (J2) or the module
+3.3 V, GND -> GND. A breakout with its own pull-ups is assumed; the pins are
+configured with the internal pull-ups as a fallback.
+
+The bus is registered as ``/dev/i2c0`` and the sensor as ``/dev/temp0``
+(``gd32_bringup()`` registers it at address 0x44). Confirm the address with
+the i2ctool using a zero-byte write probe (``-z``); the SHT3x NACKs the
+default one-byte read probe when it has no measurement pending, so plain
+``i2c dev`` would not show it::
+
+  nsh> i2c dev -z 0x03 0x77
+       0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
+  00:          -- -- -- -- -- -- -- -- -- -- -- --
+  10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+  20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+  30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+  40: -- -- -- -- 44 -- -- -- -- -- -- -- -- -- -- --
+  50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+  60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+  70: -- -- -- -- -- -- -- --
+
+The ``44`` above is the SHT3x acknowledging. Then read it through the
+driver::
+
+  nsh> sht3x
+  Converting...
+  Temperature = 26.530098
+  Humidity    = 47.998001
+
+.. note::
+   If the sensor sits at 0x45, change the address passed to
+   ``sht3x_register()`` in ``gd32_bringup.c`` (the i2ctool still finds it at
+   its real address regardless).
+
 littlefs
 --------