The imx-media driver currently supports a device tree graph of
limited complexity. This patch is a first step in allowing imx-media
to work with more general OF graphs.

The CSI subdevice assumes the originating upstream subdevice (the
"sensor") is connected directly to either the CSI mux or the MIPI
CSI-2 receiver. But for more complex graphs, the sensor can be distant,
with possible bridge entities in between. Thus the sensor's bus type
could be quite different from what is entering the CSI. For example
a distant sensor could have a parallel interface, but the stream
entering the i.MX is MIPI CSI-2.

To remove this assumption, get the entering bus config from the entity
that is directly upstream from either the CSI mux, or the CSI-2 receiver.
If the CSI-2 receiver is not in the enabled pipeline, the bus type to the
CSI is parallel, otherwise the CSI is receiving MIPI CSI-2.

Note that we can't use the direct upstream source connected to CSI
(which is either the CSI mux or the CSI-2 receiver) to determine
bus type. The bus entering the CSI from the CSI-2 receiver is a 32-bit
parallel bus containing the demultiplexed MIPI CSI-2 virtual channels.
But the CSI and its IDMAC channels must be configured based on whether
it is receiving data from the CSI-2 receiver or from the CSI mux's
parallel interface pins.

The function csi_get_upstream_endpoint() is used to find this
endpoint. It makes use of a new utility function
imx_media_find_upstream_pad(), that if given a grp_id of 0, will
return the closest upstream pad from start_entity.

With these changes, imx_media_find_sensor() is no longer used and
is removed. As a result there is also no longer a need to identify
any sensor or set the sensor subdev's group id as a method to search
for it. So IMX_MEDIA_GRP_ID_SENSOR is removed. Also the video-mux group
id IMX_MEDIA_GRP_ID_VIDMUX was never used so that is removed as well.
The remaining IMX_MEDIA_GRP_ID_* definitions are entities internal
to the i.MX.

Another use of imx_media_find_sensor() in the CSI was to call the
sensor's g_skip_frames op to determine if a delay was needed before
enabling the CSI at stream on. If necessary this will have to be
re-addressed at a later time.
Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>