ARM: dts: imx6sx: correct i.MX6sx sdb board enet phy address

The commit (3d125f9c) cause i.MX6SX sdb enet cannot work. The cause is
the commit add mdio node with un-correct phy address.

The patch just correct i.MX6sx sdb board enet phy address.

V2:
* As Shawn's suggestion that unit-address should match 'reg' property, so
  update ethernet-phy unit-address.

Acked-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>