audioinjector-octo: Add continuous clock feature

By user request, add a switch to prevent the clocks being stopped when
the stream is paused, stopped or shutdown. Provide access to the switch
by adding a 'non-stop-clocks' parameter to the audioinjector-addons
overlay.

See: https://github.com/raspberrypi/linux/issues/2409



Signed-off-by: Phil Elwell <phil@raspberrypi.org>