ARM: dts: qcom-msm8974: add SoC specific compat string to mdp5 node

Add SoC-specific compat string to the MDP5 device node to ease
distinguishing between various platforms.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230109050152.316606-4-dmitry.baryshkov@linaro.org