pmc: Add Hygon Dhyana support.

To make the PMC tool pmcstat working properly on Hygon platform, add
support for Hygon Dhyana family 18h by using the PMC initialization
code path of AMD family 17h.

Submitted by: Pu Wen <puwen@hygon.cn>
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D23562