Input: imx_keypad - add COMPILE_TEST support

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

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1583137573-16628-1-git-send-email-Anson.Huang@nxp.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>