Add PA11 power-hold output (PMOS self-latch keep-alive)

Src/pwr_hold.c

@@ -0,0 +1,23 @@
+#include "pwr_hold.h"
+#include "gd32f4xx_rcu.h"
+#include "gd32f4xx_gpio.h"
+#include "board_config.h"
+
+void pwr_hold_init(void) {
+    /* 使能 GPIO 时钟，推挽输出高（PMOS 导通，维持供电） */
+    rcu_periph_clock_enable(PWR_HOLD_RCU);
+    gpio_mode_set(PWR_HOLD_PORT, GPIO_MODE_OUTPUT, GPIO_PUPD_NONE, PWR_HOLD_PIN);
+    gpio_output_options_set(PWR_HOLD_PORT, GPIO_OTYPE_PP, GPIO_OSPEED_50MHZ, PWR_HOLD_PIN);
+    gpio_bit_set(PWR_HOLD_PORT, PWR_HOLD_PIN);
+}
+
+void pwr_hold_on(void) {
+    gpio_bit_set(PWR_HOLD_PORT, PWR_HOLD_PIN);
+}
+
+void pwr_hold_off(void) {
+    /* 软关机：置低后系统失电 */
+    gpio_bit_reset(PWR_HOLD_PORT, PWR_HOLD_PIN);
+    while (1) {
+    }
+}