platform: move internal headers to internal/ ...

They belong to internal folder to follow our guideline, not in source as they were.

Add Chrono support to Kernel and SysTimer

rtos: fix SPDX identifiers

rtos: add SPDX identifiers

Move source files and add Doxygen labels

Drivers/Events/RTOS Public and internal APIs cleanup (#10955) ...

Separate drivers, events, and rtos internal APIs from public APIs.

* Move source files to source subdirs
* Move internal headers to internal subdirs
* Add Doxygen comments for documenting internal and public APIs
* Remove source code from header files in order to remove include pre-processor directives
that included header files not directly used by said header files
* Explicitly include header files instead of implicit inclusions via third-party header files.

Release Notes

This will break user code that was using an internal API as the internal header files have been moved.
This will only break if the user was including the header file using a namespace (i.e #include "foo/bar.h" instead of #include "bar.h"