sunxi: setup simplefb for Allwinner DE2

As the support of EFI boot on Allwinner H3 is broken, we still need to
use simplefb to pass the framebuffer to Linux.

Add code to setup simplefb for Allwinner DE2 driver.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>