Add device tree support for ov2640.
In device tree, user needs to provide the master clock (xvclk).
User can add the reset/pwdn pins if they have.

Cc: devicetree@vger.kernel.org
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>