arm64: dts: allwinner: a64: add device tree node for HDMI simplefb

As the U-Boot bootloader now is also capable of initialize the HDMI on
A64 boards, add a simplefb device tree node for accessing the HDMI
framebuffer initialized by the bootloader.

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