dev-arm: Refactor GenericTimer

dev-arm: Refactor GenericTimer

The GenericTimer specification includes a global component for
a universal view of time: the System Counter.

If both per-PE architected and memory-mapped timers are instantiated
in a system, they must both share the same counter. SystemCounter is
promoted to be an independent SimObject, which is now shared by
implementations.

The SystemCounter may be controlled/accessed through the memory-mapped
counter module in the system level implementation. This provides
control (CNTControlBase) and status (CNTReadBase) register frames. The
counter module is now implemented as part of GenericTimerMem.

Frequency changes occur through writes to an active CNTFID or to
CNTCR.EN as per the architecture. Low-high and high-low transitions are
delayed until suitable thresholds, where the counter value is a divisor
of the increment given the new frequency.
Due to changes in frequency, timers need to be notifies to be
rescheduled their counter limit events based on CompareValue/TimerValue.
A new SystemCounterListener interface is provided to achieve
correctness.

CNTFRQ is no longer able to modify the global frequency. PEs may
use this to modify their register view of the former, but they should
not affect the global value. These two should be consistent.

With frequency changes, counter value needs to be stored to track
contributions from different frequency epochs. This is now handled
on epoch change, counter disable and register access.

References to all GenericTimer model components are now provided as
part of the documentation.

VExpress_GEM5_Base is updated with the new model configuration.

Change-Id: I9a991836cacd84a5bc09e5d5275191fcae9ed84b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25306
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>