gpio-rcar: Add DT support

Add DT bindings for the gpio-rcar driver and read the device
configuration from the DT node at probe time if available.

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