net: change arp header struct declaration

Changed MAC address type from uint8_t[6] to struct ether_addr and IP
address type from uint8_t[4] to uint32_t to make it consistent with
other DPDK code using MAC and IP addresses. It allows us to use
is_same_ether_addr and ether_addr_copy functions on MAC addresses in ARP header.  Also
removed union from arp_hdr struct to make calls to arp_data items
shorter. Updated test-pmd to match new arp_hdr version.

Signed-off-by: Maciej Gajdzica <maciejx.t.gajdzica@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>
[Thomas: doxygenize comments]