drm: Xilinx DRM KMS driver

The Xilinx DRM KMS driver supports the display pipelines that consist of
Xilinx Soft IPs(vdma, rgb-to-yuv convertor, chroma resampler,
video timing controller, and on-screen display) and
some IPs(adv7511 HDMI transmitter and si570) on Xilinx boards.

The multi-plane support is available with an On-Screen Display(OSD) IP,
and the number of planes depends on the OSD configuration.

Signed-off-by: Hyun Kwon <hyunk@xilinx.com>