uart: refactor internal bits from public API header file

This refactors internal bits out of the public API header file
and into its own file. This includes:

* Driver API struct which is only used by driver implementations
  but not by users of API.
* Inline implementations of various APIs.
* Inline z_impl_*.

These are implementation details that do not need to live within
the public API header file. This de-clutters the public header
file.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>