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>

xorg.conf

+# This is a minimal sample config file, which can be copied to
+# /etc/X11/xorg.conf in order to make the Xorg server pick up
+# and load xf86-video-sunxifb driver installed in the system.
+#
+# When troubleshooting, check /var/log/Xorg.0.log for the debugging
+# output and error messages.
+#
+# Run "man sunxifb" to get additional information about the extra
+# configuration options for tuning the driver.
+
+Section "Device"
+        Identifier      "Allwinner A10/A13 FBDEV"
+        Driver          "sunxifb"
+        Option          "fbdev" "/dev/fb0"
+EndSection