The ARM Cortex Microcontroller Software Interface Standard (CMSIS) defines a
set of standard interfaces to ARM Cortex family SOCs. In particular, the
CMSIS-CORE component standardizes the software interface to core and peripheral
registers, as well as exception names and the system clock frequency. Multiple
SOC vendors, including NXP and Nordic Semiconductor, include the CMSIS-CORE
header files in their SOC header files. These SOC header files are in turn used
by the vendor's peripheral drivers.

http://www.arm.com/products/processors/cortex-m/cortex-microcontroller-software-interface-standard.php

The sources in this directory are imported from
https://github.com/ARM-software/CMSIS_5.git

The current version supported in Zephyr is
https://github.com/ARM-software/CMSIS_5/releases/tag/5.6.0