luci-mod-network: add DHCPv6 NTP server config helper to interfaces

Appears under DHCPv6-service when set to 'server' or 'hybrid'.

Option 'ntp' is serviced by odhcpd. It sends a DHCPv6 NTP Server option
56 for each 'ntp' list entry.

Once uci loads the system section, it provides suggestion presets of
system configured upstream NTP servers, and the LAN IP(v6) of the router
which are then statically configured by the user.

It would make more sense if odhcpd fetches them directly.
Alas, it does not (yet).

Signed-off-by: Paul Donald <newtwen+github@gmail.com>