[media] of: move common endpoint parsing to drivers/of

This patch adds a new struct of_endpoint which is then embedded in struct
v4l2_of_endpoint and contains the endpoint properties that are not V4L2
(or even media) specific: the port number, endpoint id, local device tree
node and remote endpoint phandle. of_graph_parse_endpoint parses those
properties and is used by v4l2_of_parse_endpoint, which just adds the
V4L2 MBUS information to the containing v4l2_of_endpoint structure.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>