arm64: add a driver for the Apple Interrupt Controller

Some limited support for later multi-die SoC is included, but not at all
tested and not expected to be functional yet. kevans needs to finish
getting his serial boards constructed, as the beefiest AS machine that
actually has multiple die to support in his fleet is currently a
dedicated serial console.

Reviewed by: andrew
Co-authored-by: Andrew Turner <andrew@FreeBSD.org>
Co-authored-by: Mike Karels <karels@FreeBSD.org>
Differential Revision: https://reviews.freebsd.org/D48079