base: Correct a small typo in sim/core.(hh|cc).

base: Correct a small typo in sim/core.(hh|cc).

The value GHz with a small z was spelled GHZ with a large z, which was
inconsistent with the other frequency-like values in that namespace.

Change-Id: I55dfc447a5811ae584e46769cd9cadd08bd1e716
Reviewed-on: https://gem5-review.googlesource.com/12572
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>