mx31: add camera device

This adds the camera device based on Guennadi's soc_camera architecture
for the mx31.

The proposed init by Guennadi was removed and must be implemented in the
board init file as it contains a lot of board specific info (memory
size, clock speed).

Signed-off-by: Valentin Longchamp <valentin.longchamp@epfl.ch>
Reviewed-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>