dev-arm: GenericTimer, freq as 32-bit value

dev-arm: GenericTimer, freq as 32-bit value

The System Counter frequency is now a 32-bit value. This is consistent
with CNTFRQ and CNTFRQ_EL0 register sizes.

Change-Id: I39886a3767adbe9c58887b8b6d5f30ebc6035bcc
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22424
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>