Block secondary ITS instances from attaching on ARM64

Currently FreeBSD supports only single PIC controller. Some systems
that have more than one (like ThunderX dual-socket) fails to boot.
Disable other PICes until proper handling is implemented in the
generic interrupt code.

Reviewed by: imp
Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3682