Provide xorg.conf needed by the instructions from http://linux-sunxi.org

The installation instructions from http://linux-sunxi.org/Binary_drivers wiki
page currently ask the users to run the following command after compiling
and installing the ddx driver (either xf86-video-mali or xf86-video-sunxifb):

    cp xorg.conf /usr/share/X11/xorg.conf.d/99-mali400.conf

Regardless of whether it is a good idea to touch /usr/share/X11/xorg.conf.d
directory in the first place, providing a sample xorg.conf file may save
some users from having unnecessary troubles.

Reported-by: Michal Suchanek via https://github.com/ssvb/xf86-video-sunxifb/pull/1
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>