configs: Make the configuration of the gicv4 parameter robust

Only the GICv3 model has a `gicv4` parameter, causing the current
`baremetal.py` config to throw an exception when used with the
VExpress_GEM5_V1 platform containing a GICv2.

This patch checks for the existence of the `gicv4` parameter, allowing
all VExpress platforms to be used.

Change-Id: I72667a9caee64fa497bda516217cd424050eb242
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/69685
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

configs/example/arm/baremetal.py

@@ -142,7 +142,8 @@ def create(args):
     system.auto_reset_addr = True
 
     # Using GICv3
-    system.realview.gic.gicv4 = False
+    if hasattr(system.realview.gic, "gicv4"):
+        system.realview.gic.gicv4 = False
 
     system.highest_el_is_64 = True