greybus: hd: add optional cport enable and disable callbacks

Add optional cport enable and disable callbacks to the greybus host
drivers, that can be used to initialise and allocate/release resources
associated with a cport during connection setup/teardown (e.g. software
queues and hardware state).

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>