dt-bindings: msm:disp: add sc7180 DPU variant

Add a compatible string to support sc7180 dpu version.

Signed-off-by: Kalyan Thota <kalyan_t@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>