arm64: dts: qcom: add mdp_clk clock to the MDSS device

Add MDP_CLK ("core") clock to the mdss device to allow MDSS driver to
access HW_REV/etc registers.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220615135935.87381-5-dmitry.baryshkov@linaro.org