DSP: Import CMSIS-DSP 1.8.0 (CMSIS 5.7.0)

This commit imports the library source code of the CMSIS-DSP 1.8.0,
which is part of the CMSIS 5.7.0 release.

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

Origin: CMSIS_5
License: Apache-2.0
URL: https://github.com/ARM-software/CMSIS_5/tree/5.7.0


commit: a65b7c9a3e6502127fdb80eb288d8cbdf251a6f4
Purpose: Provide DSP functions for Arm devices
Maintained-by: External

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>