Input: pxa27x_keypad keep clock on as wakeup source

When the keypad is used as wake up source, the clock can not
be disabled. Or it can not detect key pressing.
If the keypad is used as wake up source, when resume back,
do not enable the clock and configure it again because the
register content is retained.

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