doc: Update on switch to CMSIS_6

Add a note to announce the switch to CMSIS_6 for Cortex-M arch.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>

Author: wearyzen

doc/hardware/arch/arm_cortex_m.rst

@@ -632,8 +632,16 @@ script as well.
 CMSIS
 *****
 
-Cortex-M CMSIS headers are hosted in a standalone module repository:
-`zephyrproject-rtos/cmsis <https://github.com/zephyrproject-rtos/cmsis>`_.
+Cortex-M CMSIS headers are provided through standalone module repositories:
+
+- **CMSIS 5**: `zephyrproject-rtos/cmsis <https://github.com/zephyrproject-rtos/cmsis>`_
+- **CMSIS 6**: `zephyrproject-rtos/CMSIS_6 <https://github.com/zephyrproject-rtos/CMSIS_6>`_
+
+Zephyr has begun transitioning to **CMSIS 6** as the default source for Cortex-M core headers.
+However, at present, Zephyr includes headers from **both** the CMSIS 6 and legacy CMSIS 5 modules.
+
+The legacy CMSIS 5 headers remain available primarily for compatibility with vendor HALs, while all
+new architecture-level development should use **CMSIS 6** headers whenever possible.
 
 :kconfig:option:`CONFIG_CPU_CORTEX_M` selects :kconfig:option:`CONFIG_HAS_CMSIS_CORE` to signify that
 CMSIS headers are available for all supported Cortex-M variants.

doc/releases/migration-guide-4.2.rst

@@ -585,6 +585,20 @@ Other subsystems
 Modules
 *******
 
+CMSIS
+=====
+
+* Cortex-M boards/socs now require the ``CMSIS_6`` module to build properly (instead of ``cmsis``
+  which was CMSIS 5.9.0).
+  If trying to build a Cortex-M board, do a ``west update`` to make sure that ``CMSIS_6`` module is
+  available before running ``west build`` or other commands.
+
+  Boards or SOCs or modules using the older ``cmsis`` module either with a local copy or via the
+  :kconfig:option:`CONFIG_ZEPHYR_CMSIS_MODULE_DIR` are requested to move to the ``CMSIS_6`` module
+  which can be accessed via the :kconfig:option:`CONFIG_ZEPHYR_CMSIS_6_MODULE_DIR` configuration.
+
+  Note: Zephyr will continue using the older ``cmsis`` module for Cortex-A and Cortex-R targets.
+
 Architectures
 *************