dev-arm: Add a GIC interrupt adaptor

dev-arm: Add a GIC interrupt adaptor

Add GIC-based interrupt adaptor implementations that support PPI
(ArmPPI) and SPI (ArmSPI) delivery. In addition to being useful for
"normal" memory-mapped devices, the PPI adaptor makes it possible to
use the same device model to generate both PPIs and SPIs (e.g., the
PMU).

Change-Id: I73d6591c168040faef2443430c4f1da10c387a2a
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2521
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>