ethdev: add API to retrieve multiple MAC addresses

ethdev: add API to retrieve multiple MAC addresses

Introduce rte_eth_macaddrs_get() to allow user to retrieve all ethernet
addresses assigned to given port.
Change testpmd to use this new function and avoid referencing directly
rte_eth_devices[].

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Feifei Wang <feifei.wang2@arm.com>