ARM: shmobile: Add DTS gpio-keys support for SW2 on Lager

Add DTS gpio-keys support for SW2 on the Lager board.
This makes the DT code match the legacy board code.

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