Add a remark on board reset code

kernel.c

@@ -30,6 +30,8 @@ static void uart_tx(uint8_t byte) {
 }
 
 static _Noreturn void board_reset(void) {
+    // NOTE: this seems to be the suggested method for
+    //       h6 board reset according to u-boot
     r_wdog->cfg = WDOG_CFG_SYS;
     r_wdog->mode = WDOG_MODE_EN;
     r_wdog->ctrl = WDOG_CTRL_KEY | WDOG_CTRL_RESTART;