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

platforms/chibios/bootloaders/tinyuf2.c

@@ -25,6 +25,10 @@
 extern uint32_t _board_dfu_dbl_tap[];
 #define DBL_TAP_REG _board_dfu_dbl_tap[0]
 
+__attribute__((weak)) void mcu_reset(void) {
+    NVIC_SystemReset();
+}
+
 __attribute__((weak)) void bootloader_jump(void) {
     DBL_TAP_REG = DBL_TAP_MAGIC;
     NVIC_SystemReset();