arm: apple: Add RTKit support

Most Apple IOPs run a firmware that is based on what Apple calls
RTKit. RTKit implements a common mailbox protocol.  This code
provides an implementation of the AP side of this protocol,
providing a function to initialize RTKit-based firmwares as well
as a function to do a clean shutdown of this firmware.

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