mailbox: apple: Bind to generic compatibles

As with other blocks, we intend to have drivers bind to generic
compatibles as long as there are no SoC-specific quirks. This allows
forward-compatibility with future SoCs.

No upstream DTs instantiate this yet, so it's still safe to make this
breaking change.

Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>