drm: xilinx: crtc: Add properties for z-order and alpha values

This patch adds z-order and alpha properties for CRTC. Those properties
are configured using the private plane of CRTC and the xilinx drm
plane functions. The properties are available when there are multiple
layers.

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