Add bus_release_resource() method to nexus on arm64

The nexus module was missing method for releasing bus resources. As a
result, it couldn't be released and the bus_release_resource() call would
return ENXIO.

Next call to bus_alloc_resource() for the same resource was returning
error, because it wasn't released previously and it was still busy.

The implementation of the nexus_release_resource() is the same as for
arm architecture.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reported-by: Greg V <greg@unrelenting.technology>
Tested-by: cperciva, Greg V <greg@unrelenting.technology>
Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.
Differential revision: https://reviews.freebsd.org/D19641