Documentation: serial: move GPIO kernel-doc to the functions

The GPIO uart functions are documented in Documentation. Move and
transform this documentation into kernel-doc directly in the code and
reference it in Documentation using kernel-doc:.

This makes it easier to update, maintain and check by the build.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220728061056.20799-3-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>