arch/arm/rtl8721dx: add shared Ameba GPIO driver.

Add a board-agnostic GPIO driver for Realtek Ameba chips, exposing pins
through the NuttX GPIO (ioexpander) upper half at /dev/gpioN.

- arch/arm/src/common/ameba/ameba_gpio.{c,h}: the shared driver, sitting
  directly on the SDK fwlib register layer.  The fwlib GPIO API it calls
  resolves at link time from the on-chip ROM symbol table, except
  GPIO_INTStatusGet/ClearEdge which are not in ROM and are compiled in
  from fwlib ram_common/ameba_gpio.c (that object also carries GPIO_Init,
  harmlessly overriding the equivalent ROM copy).  Pin interrupts are
  dispatched NuttX-natively: the port's NVIC vector is owned by NuttX
  via irq_attach, and the ISR reads and clears status through the fwlib
  GPIO_INTStatus* helpers.
- The driver keeps nothing IC-specific: the port count, the per-port
  NVIC vectors and the RCC gate bits come from a per-chip
  <ameba_gpio_chip.h> resolved on the include path, so bringing up a new
  Ameba chip only adds that header, not a change to the shared driver.
- arch/arm/src/common/ameba/Kconfig: a shared "Ameba Peripheral Support"
  menu with the AMEBA_GPIO option, sourced by each Ameba chip's Kconfig so
  it is reused across ICs (RTL8721Dx, RTL8720F, ...).
- arch/arm/src/rtl8721dx: provide ameba_gpio_chip.h, wire the driver into
  both the Make/Kconfig and CMake builds, and compile the fwlib
  ameba_gpio.c register layer into libameba_fwlib for the RTL8721Dx.
- boards/arm/rtl8721dx/pke8721daf: board pin table (rtl8721dx_gpio.c),
  bring-up registration, and a standalone gpio defconfig.
- Documentation: describe the gpio configuration and pin encoding.

tools/nxstyle: whitelist the Ameba "GPIO_" SDK ROM symbol prefix (alongside
the existing FLASH_/IPC_/... Ameba prefixes) so the vendor GPIO API's mixed-case
identifiers do not trip nxstyle, matching how the other Ameba SDK prefixes are
handled.

Assisted-by: Claude Code:claude-opus-4-8
Signed-off-by: dechao_gong <dechao_gong@realsil.com.cn>

tools/nxstyle.c

@@ -272,6 +272,7 @@ static const char *g_white_prefix[] =
   "DCache_",
   "ChipInfo_",
   "FLASH_",
+  "GPIO_",
   "IPC_",
   "LOGUART_",
   "OSC2M_",