sunxi: display: Add overscan correction

Add support for making the visual area of the framebuffer smaller and
drawing a black border around it. This is intended for use with
overscanning monitors (esp. with composite video out), to avoid part
of the picture being invisible.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Anatolij Gustschin <agust@denx.de>