Input: pxa27x_keypad add choice to set direct_key_mask

Direct keys usage may not start from KP_DKIN0, add a msk option
to configure the specifics for platforms that can skip some keys.

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>