dev-arm: Fix GICv3 overflow for INTID > 256

dev-arm: Fix GICv3 overflow for INTID > 256

SPIs can get to a maximum number of 1023, so that an uint8_t is not capable
of representing all of them.

Change-Id: I7a2c43b41ac93eabdfcf8311681240416b954177
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17631
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>