As the pl061 driver can be an interrupt controller attach it earlier in the
boot so other drivers can use it.
Use a new GPIO xref to not conflict with the existing root interrupt
controller.
Sponsored by: Innovate UK
A PL061 is a simple 8 pin GPIO controller. This GPIO device is used to
signal an internal request for shutdown on some virtual machines including
Arm-based Amazon EC2 instances.
Submitted by: Ali Saidi <alisaidi_amazon.com> (previouss version)
Reviewed by: Ali Saidi, manu
Differential Revision: https://reviews.freebsd.org/D24065