fastmodel: add interface to update system counter freq

fastmodel: add interface to update system counter freq

This CL set the cntfrq and system counter frequency at once from python
script. This aligns the fastmodel implementation to other part of gem5
CPU.

Change-Id: I78c9a7be801112844c03d2669a94d57015136d16
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40278
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>