dev-arm: GenericTimer, configurable base and low freqs

dev-arm: GenericTimer, configurable base and low freqs

Architecture states the system counter has a fixed base frequency
provided in the first entry of the frequency modes table. Optionally,
other lower frequencies may be specified in consecutive entries.

This patch adds configurable frequencies to the GenericTimer model.
The default base frequency is kept as the one that was previously
hardcoded for backwards compatibility.

The table is not recommended to be updated once the system is running.

Change-Id: Icba0b340a0eb1cbb47dfe7d7e03b547af4570c60
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22425
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>