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

[raulcxw]

Summary

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
  the fwlib ram_common/ameba_gpio.c. Pin interrupts are dispatched
  NuttX-natively: the port's NVIC vector is owned by NuttX via irq_attach, and
  the ISR reads/clears status through the fwlib helpers.
• The driver keeps nothing IC-specific: the port count, per-port NVIC vectors
  and 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.
• 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 and CMake builds, and compile the fwlib register layer
  (ameba_gpio.c) into libameba_fwlib for the RTL8721Dx.
• boards/arm/rtl8721dx/pke8721daf — board pin table (rtl8721dx_gpio.c),
  bring-up registration, and a standalone gpio defconfig.
• 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.
• Documentation: the gpio configuration and pin encoding.

Impact

• New: AMEBA_GPIO driver + pke8721daf:gpio configuration. Off by default;
  no effect on existing configurations.
• Builds under both make and CMake (the CMake support landed in arch/arm/ameba: add CMake build support for RTL8721Dx and RTL8720F #19463).

Testing

Fork CI on top of current apache/master, all green — the pke8721daf:gpio
config builds under both build systems, and the existing nsh configs for both
boards still build:

pke8721daf:gpio — make and CMake

# make
$ ./tools/configure.sh pke8721daf:gpio && make
-- Found assembler: ~/rtk-toolchain/asdk-10.3.1-4602/linux/newlib/bin/arm-none-eabi-gcc
PACK: wrote nuttx.bin
-rw-r--r-- 1 root root  517728 nuttx.bin

# CMake
$ . tools/ameba/env.sh pke8721daf
$ cmake -B build -DBOARD_CONFIG=pke8721daf:gpio -GNinja && cmake --build build
[1172/1174] Linking C executable nuttx
PACK: wrote build/nuttx.bin
-rw-r--r-- 1 root root  527456 build/nuttx.bin

Hardware-verified on the pke8721daf (RTL8721Dx) with the make build: the driver
was exercised on target through /dev/gpio* (input read, output write and pin
interrupts). The CMake build of the same config is CI-verified (above).

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb