arm: mvebu: dt: Add PHY LED support for 370-rd WAN port

The WAN port of the 370-RD has a Marvell PHY, with one LED on
the front panel.y List this LED in the device tree.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>