device-port: introduce available_group member

device-port: introduce available_group member

In some cases, the I/O connector functionality can be shared
and we cannot determine the proper purpose automatically.
We just know that something was inserted to the jack.

Introduce a group identifier (a simple string - unique
per group) which helps to determine the proper ports
for the application. The user interface may be used
to set the wanted behaviour.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>