dt-bindings: meson: add specific simplefb bindings

Similar to simple-framebuffer-sunxi, we support different display pipelines
that the firmware is free to choose from.

This documents the "amlogic,simple-framebuffer" compatible and the
"amlogic,pipeline" extension.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>