dt-bindings: display: Add apple,simple-framebuffer

Apple SoCs run firmware that sets up a simplefb-compatible framebuffer
for us. Add a compatible for it, and two missing supported formats.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hector Martin <marcan@marcan.st>