Some platforms provides a mechanism for configuring the mapping between
(one or two) DisplayPort intfs and their PHYs.

In particular SC8180X requires this to be configured, since on this
platform there are fewer controllers than PHYs.

The change implements the logic for optionally configuring which PHY
each of the DP INTFs should be connected to and marks the SC8180X DPU to
program 2 entries.

For now the request is simply to program the mapping 1:1, any support
for alternative mappings is left until the use case arrise.

Note that e.g. msm-4.14 unconditionally maps INTF 0 to PHY 0 on all
platforms, so perhaps this is needed in order to get DisplayPort working
on some other platforms as well.
Co-developed-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/600895/
Link: https://lore.kernel.org/r/20240625-dp-phy-sel-v3-1-c77c7066c454@linaro.org