mailbox: apple: Add driver for Apple mailboxes

Apple SoCs such as the M1 come with various co-processors. Mailboxes
are used to communicate with those. This driver adds support for
two variants of those mailboxes.

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