Add option to disable XV hardware overlay

Add the "XVHWOverlay" boolean xorg.conf option to make it possible
to disable the XV acceleration feature using display layers on
sunxi hardware.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Update man page and README to reflect diverse platform support

Update the man page and bring it up-to-date, reflecting the fact
that the driver also supports non-sunxi platforms. Add description
of the "XVHWOverlay" option.

Also a small update to the README for similar reasons.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>