platform: Adding arm_vsi header file

Adding arm_vsi.h header file which would be used with
SAI (Serial Audio Interface) CMSIS Driver Implementation.

arm_vsi.h file is copied from ARM-software public repo
in particular from AVH-TFLmicrospeech example and you can
find the file under VSI/include directory.

repo: https://github.com/ARM-software/AVH-TFLmicrospeech/


commit: 378cabbf9f5e763d2b22348a521e543b6dbe59ac

Signed-off-by: Ahmed.Ismail <Ahmed.Ismail@arm.com>