[Core] Add Reboot keycode to core (#15990)

platforms/avr/bootloaders/caterina.c

@@ -37,3 +37,12 @@ __attribute__((weak)) void bootloader_jump(void) {
     while (1) {
     }
 }
+
+__attribute__((weak)) void mcu_reset(void) {
+    // setup watchdog timeout
+    wdt_enable(WDTO_60MS);
+
+    // wait for watchdog timer to trigger
+    while (1) {
+    }
+}