CMSIS-NN: Future update to return types of all API's

[felix-johnny]

As a follow up to the 'Announcement' section in https://github.com/ARM-software/CMSIS_5/releases/tag/5.8.0 , a visible change for users of CMSIS-NN will be that the return types(arm_status) will change. The reason for that is that the current enum that is used is part of CMSIS-DSP which will be replaced by a CMSIS-NN specific one. This is a heads up for that change. As for time line, sometime around end of the year is the plan. Users of TensorFlow Lite Micro will not have to do anything as changes will be done to the TFLM repository as part of the change.