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>