arm64: dts: msm8996: Add display smmu node

Add device node for display smmu, aka. mdp_smmu.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Andy Gross <agross@kernel.org>