drm/bridge: tc358767: Detect bridge mode from connected endpoints in DT

The TC358767/TC358867/TC9595 are all capable of operating in multiple
modes, DPI-to-(e)DP, DSI-to-(e)DP, DSI-to-DPI. Only the first mode is
currently supported. It is possible to find out the mode in which the
bridge should be operated by testing connected endpoints in DT.

Port allocation:
port@0 - DSI input
port@1 - DPI input/output
port@2 - eDP output

Possible connections:
DPI -> port@1 -> port@2 -> eDP :: [port@0 is not connected]
DSI -> port@0 -> port@2 -> eDP :: [port@1 is not connected]
DSI -> port@0 -> port@1 -> DPI :: [port@2 is not connected]

Add function to determine the bridge mode based on connected endpoints.

Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Lucas Stach <l.stach@pengutronix.de> # In both DPI to eDP and DSI to DPI mode.
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220329085015.39159-10-marex@denx.de