greybus: interface: move route creation to interface activation

Creating and destroying a route to an interface is arguably an interface
operation and belongs with the interface code.

Add new interface_activate and interface_deactivate helpers that will be
used to activate and deactivate an interface in the new interface boot
sequence.

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