arch-arm: Allow ArmPPI usage for PMU

arch-arm: Allow ArmPPI usage for PMU

Differently from ArmSPIs, ArmPPI interrupts need to be instantiated by
giving a ThreadContext pointer in the ArmPPIGen::get() method. Since the
PMU is registering the ThreadContext only at ISA startup time, ArmPPI
generation in deferred until the PMU has a non NULL pointer.

Change-Id: I17daa6f0e355363b8778d707b440cab9f75aaea2
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/16204
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>