CMSIS-NN: Add elementwise add/mul int16 and unit tests (#1419)

* CMSIS-NN: Add elementwise add/mul int16 and unit tests

Zero out offsets for add/mul s16

Co-authored-by: Annie Tallund <annie.tallund@arm.com> and Måns Nilsson <mans.nilsson@arm.com>
diff --git a/ARM.CMSIS.pdsc b/ARM.CMSIS.pdsc
index c1c9994..8a2af8c 100644
--- a/ARM.CMSIS.pdsc
+++ b/ARM.CMSIS.pdsc
@@ -18,6 +18,7 @@
        - Support dilation for int8 depthwise convolution
        - Support for int16 depthwise conv for reference implementation including dilation
        - Support for int16 average and max pooling for reference implementation
+       - Support for elementwise add and mul int16 scalar version
       CMSIS-RTOS2:
         - RTX 5.5.4 (see revision history for details)
     </release>
@@ -2849,7 +2850,9 @@
         <file category="source" name="CMSIS/NN/Source/PoolingFunctions/arm_avgpool_s16.c"/>
         <file category="source" name="CMSIS/NN/Source/PoolingFunctions/arm_pool_q7_HWC.c"/>
         <file category="source" name="CMSIS/NN/Source/BasicMathFunctions/arm_elementwise_mul_s8.c"/>
+        <file category="source" name="CMSIS/NN/Source/BasicMathFunctions/arm_elementwise_mul_s16.c"/>
         <file category="source" name="CMSIS/NN/Source/BasicMathFunctions/arm_elementwise_add_s8.c"/>
+        <file category="source" name="CMSIS/NN/Source/BasicMathFunctions/arm_elementwise_add_s16.c"/>
         <file category="source" name="CMSIS/NN/Source/ActivationFunctions/arm_relu6_s8.c"/>
         <file category="source" name="CMSIS/NN/Source/ActivationFunctions/arm_relu_q15.c"/>
         <file category="source" name="CMSIS/NN/Source/ActivationFunctions/arm_relu_q7.c"/>