NN: Import CMSIS-NN (CMSIS 5.8.0)

This commit imports the library source code of the CMSIS-NN, which is
part of the CMSIS 5.8.0 release.

Note that this commit only imports the source code required to build
the CMSIS-NN library and does not import extraneous components such as
the build script and test suite.

Also note that only the TFL micro compliant functions are imported, see:
    https://github.com/ARM-software/CMSIS_5/tree/develop/CMSIS/NN
The legacy API's are no longer maintained and trigger compiler warnings.

Origin: CMSIS_5
License: Apache-2.0
URL: https://github.com/ARM-software/CMSIS_5/tree/5.8.0


commit: 13b9f72f212688d2306d0d085d87cbb4bf9e5d3f
Purpose: Provide NN functions for Arm devices
Maintained-by: External

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>

file import