SPM: Rename 'conn_handle_t' to 'connection_t'

'connection_t' for internal data structure, and 'handle' for
exposing to API callers.

- Basically a straight 'search and replace' is applied.
- Removed doxygen comments for internal APIs.

Signed-off-by: Ken Liu <Ken.Liu@arm.com>
Change-Id: If0282c3effb77518d970d5896dc8073665094bdd