IVGCVSW-3419 Add reference workload support for the new Stack layer

 * Added reference workload for the Stack layer
 * Added factory methods
 * Added validation support
 * Added unit tests

Signed-off-by: Matthew Jackson <matthew.jackson@arm.com>
Change-Id: Ib14b72c15f53a2a2ca152afc357ce2aa405ccc88
diff --git a/src/backends/reference/backend.mk b/src/backends/reference/backend.mk
index 411ab7e..6e1360a 100644
--- a/src/backends/reference/backend.mk
+++ b/src/backends/reference/backend.mk
@@ -62,6 +62,7 @@
         workloads/RefSoftmaxWorkload.cpp \
         workloads/RefSpaceToBatchNdWorkload.cpp \
         workloads/RefSpaceToDepthWorkload.cpp \
+        workloads/RefStackWorkload.cpp \
         workloads/RefStridedSliceWorkload.cpp \
         workloads/RefSplitterWorkload.cpp \
         workloads/RefTransposeConvolution2dWorkload.cpp \
@@ -69,6 +70,7 @@
         workloads/Rsqrt.cpp \
         workloads/SpaceToBatchNd.cpp \
         workloads/SpaceToDepth.cpp \
+        workloads/Stack.cpp \
         workloads/StridedSlice.cpp \
         workloads/StringMapping.cpp \
         workloads/Softmax.cpp \