Fix some build errors

Fix ABI differences between mbed-baremetal and mbed-os, remove PlatformMutex

Mutex::unlock - decrement _count inside lock ...

`Mutex::unlock` was decrementing the `_count` member after releasing
the mutex, which meant it was unprotected, exposing a race that
could corrupt the count.

This could lead to an assert in `ConditionVariable::wait`, which
checks that the mutex count is one.

Add Chrono support to Mutex

Change Mutex lock and unlock API return value to void

Remove Mutex deprecated API

rtos: fix SPDX identifiers

rtos: add SPDX identifiers

Mutex: _count incrementation brought back

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"