ethdev: promote sibling iterators to stable

ethdev: promote sibling iterators to stable

This API saw no update since its introduction and will help applications
like OVS ([1] and [2]) that currently look at rte_eth_devices[] to
achieve the same.

1: https://github.com/openvswitch/ovs/blob/master/lib/netdev-dpdk.c#L1285
2: https://github.com/openvswitch/ovs/blob/master/lib/netdev-dpdk.c#L1476

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ray Kinsella <mdr@ashroe.eu>