dev-arm: Mask el2_watchdog in VExpressFastmodel platform

el2_watchdog depends on SystemCounter. However, we have mask
system_counter in the platform. We should also mask the el2_watchdog
accordingly.

Change-Id: I2ed774549272438d654e0573ffe9f482a6659d37
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55306
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/arm/VExpressFastmodel.py

@@ -45,6 +45,7 @@ class VExpressFastmodel(VExpress_GEM5_Base):
     generic_timer = SubSystem()
     generic_timer_mem = SubSystem()
     sys_counter = SubSystem()
+    el2_watchdog = SubSystem()
 
     def _on_chip_devices(self):
         return [