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

Revised doxygen comments and fixed code style inconsistencies

Updated default vref to be NAN. Made vref an optional constructor arg

Changed default ADC vref configuration from driver-level to target-level

Update voltage wording to be consistent

Fix styling and fix mbed_lib.json syntax error

Add reference voltage getter and clarify some comments

Added API to reconfigure ADC reference voltage at runtime.

Added API call to return ADC reading in volts, scaled by configurable system ADC reference voltage.

AnalogIn: Add explicit pinmap support

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"