dt-bindings: Add property for DP intf/phy index

Add property "qcom,intf-index" and "qcom,phy-index" to specify
the INTF and PHY index for the DP controller.

Change-Id: I9ae3d13e3954e215b693b425aa0289b7f42cd30b
Signed-off-by: Jim Wang <jimwan@codeaurora.org>