dt-bindings: msm/dsi: Add mdp transfer time to msm dsi binding

Adds mdp transfer time to msm dsi binding

Changes in v3:
- Added Rob's R-b

Reviewed-by: Rob Herring <robh@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>
Signed-off-by: Rob Clark <robdclark@gmail.com>