arm: dts: ls1028a-qds: define the MDIO MUX

Add the device-tree structure describing the MUX in board dts.

QDS board has an on-board RGMII PHY and 4 slots for extension cards.
All these can be accessed over MDIO through a MDIO MUX controlled
over I2C.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>