arm64: dts: meson-gx: add support for simplefb

SimpleFB allows transferring a framebuffer from the firmware/bootloader
to the kernel, while making sure the related clocks and power supplies
stay enabled.

Add nodes for CVBS and HDMI Simple Framebuffers.

Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>