Add support for Cortex-A76/Neoverse-N1 to hwpmc

[andrew]

Summary

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.

[emaste]

looks reasonable to me

sys/arm64/include/armreg.h

866–874

I did not check these, assume they were transcribed correctly.

[bz]

With the same comment as emaste had; no checks that each value is accurate