Input: pxa27x_keypad - introduce pxa27x_keypad_config()

Introduce pxa27x_keypad_config() for keypad registers configuration
and remove the reg_kpc, reg_kprec from platform data structure
so that configurations of keypad registers can be centralized to a
single function.

It can also be re-used when resuming.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>