ARM: mach-shmobile: Kota2 GPIO Keys support

This patch ties in GPIO Keys support on the Kota2
board. For now the keys are used in polling mode,
but after extending the sh73a0 PFC with IRQ support
we should be able to switch to the IRQ driven driver.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>