dtb: xgene: Add MDIO node

Added mdio node for mdio driver.  Also added phy-handle
reference to the ethernet nodes.

Removed unused clock node from storm sgenet1.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Fushen Chen <fchen@apm.com>
Tested-by: Toan Le <toanle@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>