dev-arm, fastmodel: Rewrite Gic.interruptCells

dev-arm, fastmodel: Rewrite Gic.interruptCells

The affinity number (aka PPI partition) is used differently
in GICv2 and GICv3. In GICv2 it is ORed to the triggering type
(3rd cell), whereas it is encoded in the 4th cell in GICv3

Change-Id: I36e45d4ec5fb39befa1a271b531dfed2d8e56c10
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36235
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>