ARM: shmobile: lager: mark GPIO keys as wake-up sources

Enable wakeup for the GPIO keys on the Lager board.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi.px@renesas.com>
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>