dev-arm: Check for maximum number of supported PE in GICv3

This is currently set to 256

Change-Id: If7bb2847c22f29bfa0cb4ebf4a7984ee43ab4e29
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/+/18390
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/arm/Gic.py

@@ -178,3 +178,7 @@ class Gicv3(BaseGic):
         "HV maintenance interrupt."
         "ARM strongly recommends that maintenance interrupts "
         "are configured to use INTID 25 (PPI Interrupt).")
+
+    cpu_max = Param.Unsigned(256,
+        "Maximum number of PE. This is affecting the maximum number of "
+        "redistributors")