Adds bindings for Snapdragon 845 display processing unit

Changes in v2:
- Use SoC specific compatibles for mdss and dpu (Rob Herring)
- Use assigned-clocks to set initial clock frequency (Rob Herring)

Changes in v3 (all suggested by Rob Herring):
- Rename mdss_phys to mdss
- Correct description for clocks/assigned-clocks
- Rename mdp_phys to mdp
- Rename vbif_phys to vbif
- Remove redundant interrupt-parent from mdss_mdp
- Fully specify 'ranges' and use relative reg address in mdss_mdp

Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Rajesh Yadav <ryadav@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>