boards/zedboard: Fix Ethernet PHY dts node

* The Ethernet PHY on the ZedBoard is not guaranteed to be on address 7,
  instead use the phy broadcast address of 0 which is acceptable as no
  other devices are on the MDIO bus for the ZedBoard.

Reported-by: Philip Balister <philip@balister.org>
Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>