Documentation: devicetree: bindings: drm: xilinx: Add the connector type

Add the connector type property in the node. User can specify the type of
connector to use, and the value is used to initialize the drm connector in
the connector driver.

Signed-off-by: Hyun Kwon <hyunk@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>