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 <[e-mail]>
Acked-by: Neil Armstrong <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/khadas-mcu.c

···
 	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 = {