media: Documentation: Add GS_ROUTING documentation

Add documentation for VIDIOC_SUBDEV_G/S_ROUTING ioctl and add
description of multiplexed media pads and internal routing to the
V4L2-subdev documentation section.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>