ARM: shmobile: marzen: Use RCAR_GP_PIN macro

Replace hardcoded pin numbers with the RCAR_GP_PIN macro to make the
code match the documentation.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>