ARM: mach-shmobile: mackerel: Add keypad tca6408a support

This patch maps key0/key1/key2/key3 as HOME/MENU/BACK/POWER buttons
on mackerel board.

Signed-off-by: Tony SIM <chinyeow.sim.xt@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>