boards/risc-v/k210/maix-bit: Add userled support

[no1wudi]

Summary

Replace GPIO driver with userled driver for maix-bit board (K210 RISC-V).

The previous implementation used a generic GPIO driver (k210_gpio.c) to control the LEDs on pins 12 and 13. This commit replaces it with a proper userled driver (k210_userleds.c) that follows the NuttX LED driver conventions. The userled driver better reflects the real hardware connection since these pins are connected to LEDs, not general purpose GPIO.

Changes:

• Add user LED definitions in board.h (BOARD_LED1, BOARD_LED2)
• Add k210_userleds.c implementing board_userled_* functions
• Remove k210_gpio.c (GPIO driver no longer needed)
• Update CMakeLists.txt and Makefile for userled build
• Update k210_bringup.c to initialize userled instead of GPIO

The red LED on Pin 14 remains as system LED (CONFIG_ARCH_LEDS).

Impact

• Impact on user: NO - Neither CONFIG_DEV_GPIO nor CONFIG_USERLED_LOWER is enabled in default nsh defconfig
• Impact on build: NO - No changes to default configurations
• Impact on hardware: NO
• Impact on documentation: NO
• Impact on security: NO
• Impact on compatibility: YES - GPIO interface for pins 12/13 is removed (if CONFIG_DEV_GPIO was enabled)

Testing

Build Host: Linux, RISC-V toolchain
Target: maix-bit (K210)

Verified build with default nsh configuration (no LED drivers enabled):

LD: nuttx

Manual testing with CONFIG_USERLED_LOWER and CONFIG_EXAMPLES_LEDS enabled:

nsh> leds
leds_main: Starting the led_daemon
leds_main: led_daemon started

led_daemon (pid# 3): Running
led_daemon: Opening /dev/userleds
led_daemon: Supported LEDs 0x03
led_daemon: LED set 0x01
led_daemon: LED set 0x02
led_daemon: LED set 0x03
led_daemon: LED set 0x02
led_daemon: LED set 0x01
led_daemon: LED set 0x00
led_daemon: LED set 0x01
led_daemon: LED set 0x02
led_daemon: LED set 0x03
led_daemon: LED set 0x02

[linguini1]

Looks good!