video: add an option for video simplefb via DT

Add an option to indicate that the video driver should setup a SimpleFB
node that passes the video framebuffer initialized by U-Boot to the
operating system kernel.

Currently only the Allwinner DE driver uses this option, and the
definition of this option in the sunxi-common.h config header is
converted to an imply of this option from CONFIG_VIDEO_SUNXI.

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