arm64: Implement bus_describe_intr() for nexus

Prompted by a compiler warning introduced by
e582d4a2b09e ("arm64: nexus code tidy-up").

Reviewed by: mhorne, andrew
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39749

(cherry picked from commit 7623cc8f65ab1b28690d4a6580ae87b7926ed008)