Input: matrix-keypad - add device tree support

Also the driver was modifued to take advantage of recent improvements in
matrix_keypad_build_keymap() implementation, which automatically allocates
memory for keymap.

The driver was tested on AM335x EVM.

Signed-off-by: AnilKumar Ch <anilkumar@ti.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>