Update of CMSIS and CMSIS-RTX to latest released version and newlly as submodules

[JohnK1987]

Summary of changes

The PR brings update of CMSIS 6 Core and RTX 5.9. This is now done using submodules, which will allow easy update in future because the implementation is done cleanly via CMake without modifying submodules.
Like before it supports both bare‑metal and full RTOS profiles with shared RTOS2 API headers; kernel sources only link in full profile.
Some parts were also prepared for future possibility to add FreeRTOS as alternative RTOS backend.

Impact of changes

• CMSIS 6 supports Cortex-M85
• Future update can be more easy

Migration actions required

Proper testing on different targets. Because there were done some changes like

• Some names of NVIC_Type were unified across different cortex types
• Some CMSIS core register names were unified across different cortex types

Documentation

The cmsis readme file was updated and simplified

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[x] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

I did GT test with Nucleo-H743ZI2 before and after implementation if these changes and results were same.

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Reviewers

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