ARM: pxa: add gpio keys information

Switching from corgikbd to matrixkbd corgi has lost support for few switches.
Readd them through gpio-keys-polled device (-polled, since scoop does
not provide IRQ for GPIO).

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>