ARM: dts: socfpga: Add missing PHY phandle

Add missing PHY phandle into the DT, otherwise the stmmac code won't
detect the PHY correctly anymore.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>