documentation: dt: vtpg: Add support for max width and height

documentation: dt: vtpg: Add support for max width and height

Adds two optional parameters xlnx,max-width and xlnx,max-height which
determine the max size of frame the IP can process.

Signed-off-by: Vishal Sagar <vishal.sagar@xilinx.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>