greybus: camera: Register capabilities operation

Register the greybus camera driver capabilities operation to the
ara_camera subdevice

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>