dt-bindings: msm/mdp4: Create a separate binding doc for MDP4

MDP4 and MDP5 vary a bit in terms of device hierarchy and the properties
they require. Rename the binding doc to mdp4.txt and remove MDP5 specific
pieces. A separate document will be created for MDP5

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>