dt-bindings: Add property to force DSI TX simulate CSI signal

Add new property "qcom,csi-proxy-enable". When enabled, it will force
DSI transmitter changes the packet DataTypes to simulate the CSI-2
compatible signal.

Change-Id: I9e77845c67cd7b1a9118261488ce411fd233858f
Signed-off-by: Jim Wang <jimwan@codeaurora.org>