Input: imx_keypad - add pm suspend and resume support

The imx_keypad driver was indicating that it was wakeup capable in
imx_keypad_probe(), but it didn't implement suspend or resume methods.

According to the i.MX series MCU Reference Manual, the kpp (keypad
port) is a major wake up source which can detect any key press even
in low power mode and even when there is no clock.

Signed-off-by: Hui Wang <jason77.wang@gmail.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>