ethdev: add siblings iterators

ethdev: add siblings iterators

If multiple ports share the same hardware device (rte_device),
they are siblings and can be found thanks to the new functions
and loop macros.
One iterator takes a port id as reference,
while the other one directly refers to the parent device.

The ownership is not checked because siblings may have
different owners.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Tested-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>