dt-bindings: mailbox: Add Apple mailbox bindings

Apple mailbox controller are found on the M1 and are used for
communication with various co-processors.

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>