greybus: svc: make svc a device

Make the svc object a child device of the host-device device and use
driver core to manage its lifetime.

The svc device name is "<bus_id>-svc", where bus_id is the dynamically
assigned id of the host device.

Note that there is exactly one svc-device per host device (bus).

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