Input: imx_keypad - reset the hardware before enabling

Ensure the hardware is correctly initialized before requesting the
interrupt, otherwise if a key was already touched since power-on the
kernel enters an interrupt loop. To fix this issue we clear pending
interrupt sources. We also have to make sure clk is enabled while
changing the keypad registers.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>