net: arp: Make arp_update() function externally visible

So that it's possible to register ARP entries manually. Needed for DHCP
server implementation, which in unicast mode needs to reply to an IP
address that is not registered on the peer interface yet (hence no ARP
reply will be sent). It's needed to add an ARP entry manually in that
case, as hardware address is already known at that point.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>