Add bus_release_resource() method to nexus on arm64

[mk_semihalf.com]

Summary

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

Test Plan

ifconfig up/down/up on any interface or reload of an module

[val_packett.cool]

I can confirm that mtu changes on ena work fine with this patch, nice

[cperciva]

Works for me. Please commit, unless anyone more familiar with ARMy things says there's a reason why this code was missing...