dt-bindings: mailbox: apple,mailbox: Add power-domains property

This will bind to the PMGR pwrstate nodes that control power/clock
gating to SoC blocks. The mailbox driver doesn't do runtime-pm yet, so
initially this will just keep the domain on permanently.

Reviewed-by: Sven Peter <sven@svenpeter.dev>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>