Input: ep93xx_keypad - cleanup and use matrix_keypad helpers

Use struct matrix_keymap_data to supply the keymap from the platform code
and matrix_keypad_build_keymap() to initialize the keymap.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>