dt-bindings: msm/dsi: Modify port and PHY bindings

The DSI node now has two ports that describe the connection between the
MDP interface output and the DSI input, and the connection between the DSI
output and the connected panel/bridge. Update the properties and the
example.

Also, use generic PHY bindings instead of the custom one.

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