fastmodel: Adopt the gem5 namespace

Change-Id: I2317a2593fafcce26a30d8d7b900e844daa64714
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52263
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/fastmodel/common/signal_sender.hh

@@ -35,7 +35,11 @@
 
 #include "dev/intpin.hh"
 
-namespace FastModel
+namespace gem5
+{
+
+GEM5_DEPRECATED_NAMESPACE(FastModel, fastmodel);
+namespace fastmodel
 {
 
 class SignalSender : public IntSinkPinBase
@@ -53,6 +57,7 @@ class SignalSender : public IntSinkPinBase
     void lowerOnDevice() override { signal_out.set_state(id, false); }
 };
 
-} // namespace FastModel
+} // namespace fastmodel
+} // namespace gem5
 
 #endif  // __ARCH_ARM_FASTMODEL_COMMON_SIGNAL_SENDER_HH__