Input: max7359_keypad - switch to using matrix_keypad_build_keymap()

max7359_build_keycode() does the same thing as matrix_keypad_build_keymap(),
but the latter can also handle DT bindings.

Tested on beagleboard-xm.

Signed-off-by: Evgeniy A. Dushistov <dushistov@mail.ru>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>