greybus: interface: implement interface activation and power down

Implement the interface activation and power-down sequences.

Note that all the required SVC operations have not yet been implemented
so some stub functions are used for now.

Support for hibernating the UniPro link depends on future
power-management work so a stub function is used also for this.

Interface type handling will be refined later.

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