dt-bindings: display: xlnx: Adding dsi bindings

dt-bindings: display: xlnx: Adding dsi bindings

Adding device tree documentation of dsi.
The Xilinx MIPI DSI (Display serial interface) Transmitter Subsystem
implements the Mobile Industry Processor Interface (MIPI) based display
interface. The Display Serial Interface Specification defines protocols
between a host processor and peripheral devices using a D-PHY physical
interface. The DSI specification builds on existing specifications by
adopting pixel formats and command set defined in MIPI Alliance
specifications for Display Pixel Interface 2 (DPI-2) and Display
Command Set (DCS)

Signed-off-by: Saurabh Sengar <saurabhs@xilinx.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>