The board has 3 LEDs connected to GPIOs. Add a led-gpio device to
support them.

Based on "ARM: shmobile: marzen: Add GPIO LEDs" by Laurent Pinchart.

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