net: openthread: handle non mesh-local IPv6 addresses as DHCPv6

OpenThread BR can assign addresses via DHCPv6 or when acting as an
NCP, addresses can be added manually. Currently, those addresses are
handled in the same way as auto-configured addresses.

This patch maps the newly introduced mAddressOrigin of otNetifAddress to
Zephyr's net_addr_type.

This way an application can register a handler and differentiate by type
of assignment:

```
static void handler(struct net_mgmt_event_callback *cb,
                    u32_t mgmt_event,
                    struct net_if *iface)
{
  if (iface->config.ip.ipv6->unicast[i].addr_type == NET_ADDR_DHCP) {
  }
}
```

Signed-off-by: Markus Becker <markus.becker@tridonic.com>