dt-bindings: simplefb-sunxi: Add sunxi simplefb extensions

If pre-filled framebuffer nodes are used, the firmware may need extra
properties to find the right node. This documents the properties to use
for this on sunxi platforms.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>