Add option to disable XV hardware overlay, and update driver documentation

These are two relatively minor commits. The first adds an option to disable the accelerated XV support on sunxi hardware. The boolean option is called "XVHWOverlay". This is good for completeness and could be useful in cases where the accelerated XV support is problematic (hardware with too few available layers, untested chips etc).

The second makes the documentation more up-to-date, also reflecting the fact the driver supports more platforms besides sunxi, and adds a description of the "XVHWOverlay" option. See also https://github.com/ssvb/xf86-video-sunxifb/issues/10