greybus: Add driver for the camera class protocol

Integration with the V4L2 camera drivers isn't available yet, a debugfs
interface is exposed instead to call the camera Greybus operations.

The debugfs interface will be kept for module testing purpose in order
to exercise all the protocol operations with various valid and invalid
parameters.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo.mondi@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>