arm64: dts: qcom: sm6350: correct DP compatibility strings

arm64: dts: qcom: sm6350: correct DP compatibility strings

SM6350 doesn't have MST support, as such in DT schema it has been
switched to use SC7180 as a fallback compatible. Make DT file implement
this change. DisplayPort on SC7180 has been supported long ago (and long
before we added support for DP on SM8350). The driver will continue to
work with the old DTS (having qcom,sm8350-dp fallback compatible) as
even after adding MST support the driver will have to support old SM8350
DTS which didn't have MST clocks.

Fixes: 62f87a3cac4e ("arm64: dts: qcom: sm6350: Add DisplayPort controller")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250916-dp_mst_bindings-v9-1-68c674b39d8e@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>