arm: dts: lx2160aqds: add MDIO slots

arm: dts: lx2160aqds: add MDIO slots

The LX2160A processor has two external MDIO interfaces, described in the
DTS as emdio1 and emdio2.

On the LX2160AQDS board EMDIO1 is used with two onboard RGMII PHYs
(Realtek RTL8211FD-CG), as well as eight input/output connectors for
mezzanine cards. Configuration signals from the Qixis FPGA control the
routing of the external MDIOs.

Register 0x54 of the Qixis FPGA controls the routing of the EMDIO1 one
of the 8 IO slots. As a consequence, a new node is added to
describe register 0x54 as a MDIO mux controlled with child nodes
describing all the IO slots as MDIO buses.

Also, DPMAC 17 and 18 are updated to reference the on-board PHYs.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Razvan Ionut Cirjan <razvanionut.cirjan@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>