ARM: dts: imx6: RDU2: Add Micrel PHY to FEC

The FEC has a Micrel PHY connected to it. This PHY is managed using
the bit-banging MDIO bus. Add this to the device tree.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Chris Healy <cphealy@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>