dev-arm: Fix GICv3 IGRPMOD writes

dev-arm: Fix GICv3 IGRPMOD writes

Writes to IGRPMOD were not right shifting the value, which resulted in
interrupts having a IGRPMOD value > 1, whereas the only allowed values
are 0 and 1.

Change-Id: Id491bd1b184d6e5abeeea25ea272eeb91792ccf7
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/public/gem5/+/20613
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>