[media] soc_camera: add support for dt binding soc_camera drivers

Add initial support for OF based soc-camera devices that may be used
by any of the soc-camera drivers. The driver itself will need converting
to use OF.

These changes allow the soc-camera driver to do the connecting of any
async capable v4l2 device to the soc-camera driver. This has currently
been tested on the Renesas Lager board.

It currently only supports one input device per driver as this seems
to be the standard connection for these devices.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[g.liakhovetski@gmx.de add check for multiple subdevices]
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Tested-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>