dt-bindings: display: Fix simple-framebuffer example

The simple-framebuffer binding has a compatible that isn't one of the valid
options. Since an Allwinner pipeline is being described, let's add the
matching compatible.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>