hwpmc_arm64: fill kern.hwpmc.cpuid

This will be used to detect supported pmu events. The expected format is
the MIDR register with the revision and variant fields masked. See also:
lib/libpmc/pmu-events/arch/arm64/mapfile.csv.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30601