arch-arm: Add an Armv8.4 ArmRelease object

Change-Id: I0fc4301be2b4f3fa29c4320ab747bfa88132d434
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51021
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/arch/arm/ArmSystem.py

@@ -134,6 +134,11 @@ class Armv83(Armv82):
         'FEAT_FCMA', 'FEAT_JSCVT', 'FEAT_PAuth',
     ]
 
+class Armv84(Armv83):
+    extensions = Armv83.extensions + [
+        'FEAT_SEL2'
+    ]
+
 class ArmSystem(System):
     type = 'ArmSystem'
     cxx_header = "arch/arm/system.hh"