net: ipv6: Adding router mgmt add/del events

If IPv6 router is added, then NET_EVENT_IPV6_ROUTER_ADD event
is generated. When router is removed, NET_EVENT_IPV6_ROUTER_DEL
is then generated. This is useful info if we want to have a generic
connectivity to outside of our local network.

Change-Id: Ia03958a071ceb998127894025c99ab72a8b648d5
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>