net: ipv6: Introduce separate logger module for IPv6 ND

Debugging IPv6 Neighbor Discovery issues requires to enable full IPv6
logs, which can get bloated given it provides logs for every single
packet. We should be able to focus on IPv6 ND logs only, hence introduce
a separate log module for IPv6 ND.

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