CMSIS-NN : MVE int16 average pooling support (#1540)

* CMSIS-NN : MVE int16 average pooling support

MVE optimization is added for int16 average pooling.

New test cases are added to increase code coverage
diff --git a/ARM.CMSIS.pdsc b/ARM.CMSIS.pdsc
index 33422d5..a43c1ee 100644
--- a/ARM.CMSIS.pdsc
+++ b/ARM.CMSIS.pdsc
@@ -16,7 +16,7 @@
        - Support for int16 average pooling DSP implementation
        - Support for DSP extension optimization for int16 depthwise_conv
        - Support for MVEI extension optimization for int16 depthwise_conv
-       - Support for MVEI extension optimization for int16 max pooling
+       - Support for MVEI extension optimization for int16 max and average pooling
     </release>
     <release version="5.9.0" date="2022-05-02">
       CMSIS-Core(M): 5.6.0