drm: xilinx: osd: Add the OSD_MAX_ALPHA value

Add the OSD_MAX_ALPHA value. This defines the fixed max alpha value of OSD
for 8 bit video component width. The better way to handle this by having
the video component width property in OSD DT node, and configuring
the value based on those properties.

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