samples: net: dsa: Enable DHCPv4 and start it when interface is up

Enable DHCPv4 when network interface goes up. This makes it easier
to test IP connectivity with DSA.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>