mfd: khadas-mcu: Fix randconfig 'unused-const-variable' warning

When testing with !OF, the build system reports:

>> drivers/mfd/khadas-mcu.c:125:34: warning: unused variable 'khadas_mcu_of_match' [-Wunused-const-variable]
   static const struct of_device_id khadas_mcu_of_match[] = {
                                    ^
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/khadas-mcu.c

@@ -122,11 +122,13 @@ static int khadas_mcu_probe(struct i2c_client *client,
 	return 0;
 }
 
+#ifdef CONFIG_OF
 static const struct of_device_id khadas_mcu_of_match[] = {
 	{ .compatible = "khadas,mcu", },
 	{},
 };
 MODULE_DEVICE_TABLE(of, khadas_mcu_of_match);
+#endif
 
 static struct i2c_driver khadas_mcu_driver = {
 	.driver = {