Input: pxa27x_keypad direct key may be low active

KPDK_DK only indicates the pin level of direct key.
So it is related to board, and low level may be active which
indicates that a key is pressed.

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