mailbox: apple: Add driver for Apple IOP mailbox

This mailbox driver provides a communication channel with the
Apple IOP controllers found on Apple SoCs.  These IOP controllers
are used to implement various functions such as the System
Manegement Controller (SMC) and NVMe storage.  It allows sending
and receiving a 96-bit message over a single channel.

The header file with the struct used for mailbox messages is taken
straight from Linux.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Sven Peter <sven@svenpeter.dev>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested on: Macbook Air M1
Tested-by: Simon Glass <sjg@chromium.org>