video: Add Video Pipeline (VPL) support ...

Complex video pipelines are modelled with the of_graph bindings in
the devicetree. This patch adds a ioctl infrastructure to issue
commands to the remote endpoint of a of_graph. Currently defined
ioctls are prepare/unprepare, enable/disable and get_modes. This
is enough to control LVDS or HDMI encoder or simple panels.

A device node which contains of_graph endpoints can be registered
as a VPL entity. An entity can receive ioctls via the .ioctl callback
and also issue ioctls by calling vpl_ioctl. The core itself will never
iterate over the entire pipeline. Instead, the different VPL entities
should forward an ioctl to the next instance in the pipeline whenever
necessary.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>