ARM: dts: imx6: RDU2: Add DSA support for the Marvell 88E6352

The RDU2 has a Marvell 88E6352 switch. Both the FEC and the i210
Ethernet interfaces are connected to the switch. Make the FEC the DSA
"CPU" port, and the i210 as a regular port on the switch.

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>