arm64: dts: ti: k3-am64: Enable MDIO nodes at the board level

MDIO nodes defined in the top-level AM64x SoC dtsi files are incomplete
and will not be functional unless they are extended with a pinmux.

As the attached PHY is only known about at the board integration level,
these nodes should only be enabled when provided with this information.

Disable the MDIO nodes (in both CPSW and ICSSG) in the dtsi files and
only enable the ones that are actually pinned out on a given board.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20221017192532.23825-9-afd@ti.com