arm64: dts: lx2160a-qds: Add mdio mux nodes

The two external MDIO buses used to communicate with phy devices that
are external to SOC are muxed in LX2160AQDS board.  These buses can be
routed to any one of the eight IO slots on LX2160AQDS board depending on
value in fpga register 0x54.  Additionally the external MDIO1 is used to
communicate to the onboard RGMII phy devices.  The mdio1 is controlled
by bits 4-7 of fpga register and mdio2 is controlled by bits 4-7 of fpga
register.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>