arch/risc-v/k210: Add Watchdog Timer (WDT) driver support.

[no1wudi]

Summary

Add Watchdog Timer (WDT) driver support for the Kendryte K210 RISC-V SoC.

The K210 has two independent watchdog timers (WDT0 and WDT1) that can be used to detect and recover from system malfunctions. This driver implements the standard NuttX watchdog character driver interface.

Changes include:

• Add WDT driver implementation (k210_wdt.c, k210_wdt.h)
• Add hardware register definitions (hardware/k210_wdt.h)
• Add memory map definitions for WDT0 (0x50400000) and WDT1 (0x50410000)
• Add IRQ definitions for WDT0 (IRQ 21) and WDT1 (IRQ 22)
• Add Kconfig options: CONFIG_K210_WDT, CONFIG_K210_WDT0, CONFIG_K210_WDT1
• Add WDT initialization in maix-bit board bringup
• Update platform documentation

Impact

• Impact on user: YES - Enables watchdog timer functionality for K210 boards
• Impact on build: YES - Adds new driver source files and Kconfig options
• Impact on hardware: YES - Supports K210 WDT0 and WDT1 peripherals
• Impact on documentation: YES - Added WDT section to K210 platform docs
• Impact on security: YES - Watchdog improves system reliability and recovery
• Impact on compatibility: NO

Testing

Build Host: Ubuntu 22.04, x86_64, GCC 12.2.0
Target: risc-v k210:maix-bit:nsh

Runtime output:

NuttShell (NSH) NuttX-12.12.0
nsh> wdog
  ping elapsed=0
  ping elapsed=491
  ping elapsed=991
  ping elapsed=1491
  ping elapsed=1991
  ping elapsed=2491
  ping elapsed=2991
  ping elapsed=3491
  ping elapsed=3991
  ping elapsed=4491
  ping elapsed=4991
  NO ping elapsed=5491
  NO ping elapsed=5991
  NO ping elapsed=6491
  NO ping elapsed=6991
  NO ping elapsed=7491

NuttShell (NSH) NuttX-12.12.0
nsh> uptime
00:00:05 up  0:00, load average: 0.00, 0.00, 0.00
nsh>

Verified:

• Watchdog pings working (~500ms intervals)
• After ping timeout (5 missed pings), watchdog triggers system reset
• System reboots successfully after watchdog timeout