arm64: dts: meson-g12: 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, based on the GXBB/GXL/GXM
support at [1].

[1] 03b37035

 ("arm64: dts: meson-gx: add support for simplef")

Cc: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>