arm: add an interrupt rman to nexus

Allow the nexus bus to own and manage interrupt resources. Currently,
interrupt resources on this architecture are managed completely by
ofwbus, but it is desirable that system-wide memory and interrupt
resources be managed by the top-level bus.

This is a pre-requisite to moving this resource management out of
ofwbus.

Reviewed By: ian, Elliott Mitchell <ehem+freebsd@m5p.com>
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D32357