This adds support for the Cortex-A76 and Neoverse-N1 PMU counters to pmc.
While here add more PMCR_IDCODE values and check the implementers code is
correct before setting the PMU type.
Reviewed by: bz, emaste (looks reasonable to me)
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D25959