greybus: camera: Implement the capabilities operation

The operation queries the camera module for its capabilities. The
debugfs interface just prints a hex dump of the binary message.

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