arch-arm: ArmPMU refactor

arch-arm: ArmPMU refactor

Change the definition of PMU events in order to integrate events not
cannot easily be represented by probe points. The software
increment event is now defined as a special type with its separate
implementation in pmu.cc and pmu.hh.

Change-Id: I43874b9641bf38c54f6ba2c26386542b6a73e282
Signed-off-by: Jose Marinho <jose.marinho@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5764
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>