risc-v/gd32vw55x: fix ADC reopen and add gd32vw553k-start adc example

adc_shutdown() gates the ADC clock (RCU_APB2EN.ADCEN) when the device is
closed, but the one-shot adc_reset() that first enabled it only runs at
registration and adc_setup() never re-enabled it. A second open() then drove
a clock-gated peripheral whose conversions never completed, so a second run of
a reader such as the adc example hung after opening the device. adc_setup()
now re-enables the ADC clock; clock gating preserves the register
configuration, so nothing else has to be re-programmed.

Add an "adc" configuration that exercises the driver. It is the nsh base plus
the ADC driver, registered as /dev/adc0 and read by the adc example. ADC
channel 8 (ADC_IN8) is routed to PB0 on the J1 header so an analog voltage
applied there is sampled. The board bringup samples that channel when the pin
is routed, and PB0 is only claimed for the ADC when SPI is off (with SPI on it
belongs to the I2C0 fallback), so the periph configuration is unaffected.

The GD32VW55x ADC converts on demand rather than continuously, so the example
uses the software trigger (CONFIG_EXAMPLES_ADC_SWTRIG) and is bounded to 20
sample groups so it returns to the prompt.

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

arch/risc-v/src/gd32vw55x/gd32vw55x_adc.c

@@ -419,6 +419,15 @@ static int adc_setup(struct adc_dev_s *dev)
       return ret;
     }
 
+  /* Re-enable the ADC clock.  adc_shutdown() gates it on close, and the
+   * one-shot adc_reset() that first turned it on only runs at registration;
+   * without this a second open would drive a clock-gated peripheral and its
+   * conversions would never complete.  Clock gating preserves the register
+   * configuration set up by adc_reset(), so nothing has to be re-programmed.
+   */
+
+  modifyreg32(GD32VW55X_RCU_APB2EN, 0, RCU_APB2EN_ADCEN);
+
   /* Make sure that the interrupts are disabled and power up the ADC */
 
   modifyreg32(GD32VW55X_ADC_CTL0,