greybus: svc: add interface eject operation

Add a new svc operation which will be used to send a request to eject a
given interface.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Jeffrey Carlyle <jcarlyle@google.com>