gpio: mxs: add COMPILE_TEST support for GPIO_MXS

Add COMPILE_TEST support to GPIO_MXS driver for better compile
testing coverage.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>