drm: xlnx: vtc: Adding VTC as bridge driver

drm: xlnx: vtc: Adding VTC as bridge driver

The Xilinx Video Timing Controller is used to generate the
timings for Video IPs.It is implemented as a Xilinx bridge driver
connected to the CRTC like Xilinx Video Mixer.
Currently it supports only generator mode.

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>