IVGCVSW-6469 Add MirrorPad FrontEnd and Ref Support

 * Added PaddingMode enum to PaddingDescriptor to enable Symmetric and
   Reflect padding.
 * Added Symmetric and Reflect Ref implementation.
 * Added Serializer & Deserializer support.
 * Added unit tests.

Signed-off-by: Matthew Sloyan <matthew.sloyan@arm.com>
Change-Id: I4bed907b31742b32ccefe5e8ca39a6f1e5bd9dee
diff --git a/src/backends/reference/backend.mk b/src/backends/reference/backend.mk
index f8169a6..7049279 100644
--- a/src/backends/reference/backend.mk
+++ b/src/backends/reference/backend.mk
@@ -41,6 +41,7 @@
         workloads/Lstm.cpp \
         workloads/LstmUtils.cpp \
         workloads/Concatenate.cpp \
+        workloads/MirrorPad.cpp \
         workloads/Pad.cpp \
         workloads/Pooling2d.cpp \
         workloads/PreluImpl.cpp \