[media] v4l: Support s_crop and g_crop through s/g_selection

Fall back to s_selection if s_crop isn't implemented by a driver. Same for
g_selection / g_crop.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>