staging: imx-drm-core: Use OF graph to find components and connections between encoder and crtcs

This patch adds support to find the involved components connected to
the IPU display interface ports using the OF graph bindings documented
in Documentation/devicetree/bindings/media/video-interfaces.txt.
It makes use of the of_graph (formerly v4l2_of) parsing helpers and
thus depends on the patch that moves those out to drivers/of.

Each display interface needs to have an associated port node in the
device tree. We can associate this node with the crtc platform device
and use it to find the crtc corresponding to a given port node instead
of using a combination of parent device node and id number, as before.

Explicitly converting the void* cookie to the port device tree node
allows to get rid of the ipu_id and di_id fields. The multiplexer
setting on i.MX6 now can be obtained from the port id (reg property)
in the device tree.

The imx-drm node now needs a ports property that contains phandles
to each of the IPU display interface port nodes. From there, all
attached encoders are scanned and enabled encoders are added to a
waiting list.
The bind order makes sure that once all components are probed, crtcs
are bound before encoders, so that imx_drm_encoder_parse_of can be
called from the encoder bind callbacks.

For parsing the OF graph, temporary copies of the V4L2 OF graph
helpers are used, that can be removed again once those are available
at a generic place.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>