arm64: Use the common activate function in nexus

For non-memory and non-ioport spaces use the common
bus_generic_rman_activate_resource function in the arm64 nexus. We
can't currently use it for memory types as it doesn't support
non-posted memory.

Reviewed by: jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47283