dev,arm: add Kvm mode of operation for CP15 timer

dev,arm: add Kvm mode of operation for CP15 timer

The timer device exposed via the ARM ISA, also known as the
"CP15 timer" due to its legacy coprocessor encodings, is
implemented by the GenericTimerISA class.  During Kvm
execution, however, this functionality is directly emulated
by the hardware.

This commit subclasses the GenericTimer, which is (solely)
used by GenericTimerISA, to facilitate Kvm in much the same
way as the prior GIC changes: the gem5 model is used as the
backing store for state, so checkpointing and CPU switching
work correctly, but isn't used during Kvm execution.

The added indirection prevents the timer device from creating
events when we're just updating its state, but not actually
using it for simulation.

Change-Id: I427540d11ccf049c334afe318f575146aa888672
Reviewed-on: https://gem5-review.googlesource.com/3542
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>