arm64: Add support for FIQs

arm64 supports FIQs, fast interrupt requests, which are required by
Apple silicon which hardwires the ARM timers to FIQs. This is needed by
the upcoming Apple Interrupt Controller. Based on work by andrew@ and
kevans@ in https://reviews.freebsd.org/D40161.

Signed-off-by: Ayrton Munoz <a.munoz3327@gmail.com>
Co-authored-by: Kyle Evans <kevans@FreeBSD.org>
Co-authored-by: Andrew Turner <andrew@FreeBSD.org>
Reviewed-by: imp,mmel,mhorne
Pull-Request: https://github.com/freebsd/freebsd-src/pull/1363