Input: omap4-keypad - add device tree support

Add device tree support for omap4 keypad driver and update the
Documentation with omap4 keypad device tree binding information.

Tested on omap4430 sdp.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>