soc: apple: mailbox: Add ASC/M3 mailbox driver

This new driver is based on the existing apple-mailbox driver, but
replaces the usage of the mailbox subsystem with directly exported
symbols.

As part of this refactor, this adds support for using the hardware FIFOs
(not supported in mailbox) and implicitly fixes a bunch of bugs caused
by bad interactions with the mailbox subsystem. It also adds runtime-PM
support.

The new config symbol is APPLE_MBOX, while the module name remains
identical ("apple-mailbox"). The configs are mutually exclusive in
Kconfig, to avoid conflicts.

Acked-by: Eric Curtin <ecurtin@redhat.com>
Acked-by: Neal Gompa <neal@gompa.dev>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Hector Martin <marcan@marcan.st>