CMSIS-NN: Add int16 conv kernel support

* Adds test_arm_convolve_wrapper_s16 and test_arm_convolve_16 with
  reference implementation.
* This requires new requantization so it is based on new function,
  arm_nn_requantize_s64.
* Adds unit tests for this.

Change-Id: I730458996472d6dbb51d239bc7530dd53d21130f