On am642-evm the CPSW3g ext. Port1 is directly connected to TI DP83867 PHY
and Port2 is connected to TI DP83869 PHY which is shared with ICSS
subsystem. The TI DP83869 PHY MII interface is configured using pinmux for
CPSW3g, while MDIO bus is connected through GPIO controllable 2:1 TMUX154E
switch (MDIO GPIO MUX) which has to be configured to route MDIO bus from
CPSW3g to TI DP83869 PHY.

Hence add networking support for am642-evm:
- add CPSW3g MDIO and RGMII pinmux entries for both ext. ports;
- add CPSW3g nodes;
- add mdio-mux-multiplexer DT nodes to represent above topology.
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210304211038.12511-4-grygorii.strashko@ti.com