IVGCVSW-4440 : Add HARD_SWISH Support to Activation in CpuRef

* Add a new Activiation type of HardSwish.
* Add CpuRef support and tests.

Signed-off-by: Colm Donelan <Colm.Donelan@arm.com>
Change-Id: I68c3840aa45b7a27d5e416a5d50fe8f99f003ce8
diff --git a/src/armnnSerializer/Serializer.cpp b/src/armnnSerializer/Serializer.cpp
index a3fdcf8..39df0c2 100644
--- a/src/armnnSerializer/Serializer.cpp
+++ b/src/armnnSerializer/Serializer.cpp
@@ -48,6 +48,8 @@
             return serializer::ActivationFunction::ActivationFunction_Square;
         case armnn::ActivationFunction::Elu:
             return serializer::ActivationFunction::ActivationFunction_Elu;
+        case armnn::ActivationFunction::HardSwish:
+            return serializer::ActivationFunction::ActivationFunction_HardSwish;
         default:
             return serializer::ActivationFunction::ActivationFunction_Sigmoid;
     }