[ncp] Support for multicast address table (#2001)

This commit adds a new spinel property (along with its prop handlers)
`SPINEL_PROP_IPV6_MULTICAST_ADDRESS_TABLE` to provide the list of
multicast IPv6 addresses.

It also adds `OT_CHANGED_IP6_MULTICAST_SUBSRCRIBED` and
`OT_CHANGED_IP6_MULTICAST_UNSUBSRCRIBED` to flags used in
`NetifCallback` to indicate changes in multicast address list.

doc/spinel-protocol-src/spinel-prop-ipv6.md

@@ -22,6 +22,7 @@ IPv6 Prefix + Prefix Length
 * Type: Read-Write
 * Packed-Encoding: `A(t(6CLLC))`
 
+This property provides all unicast addresses.
 Array of structures containing:
 
 * `6`: IPv6 Address
@@ -39,3 +40,10 @@ turned on, ping request ICMP packets will not be passed to the host.
 
 Default value is `false`.
 
+### PROP 102: SPINEL_PROP_IPV6_MULTICAST_ADDRESS_TABLE {#prop-ipv6-multicast-address-table}
+* Type: Read-Write
+* Packed-Encoding: `A(t(6))`
+
+Array of structures containing:
+
+* `6`: Multicast IPv6 Address