ARM: shmobile: Enable DSW2 with gpio-keys on KZM9D

Use the gpio-keys driver to support the 4 pins on the
dip switch DSW2 which is mounted on the KZM9D board.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>