arm64: dts: ti: k3-j721e-main: Add ICSSG MDIO nodes

The ICSSGs on K3 J721E SoCs contain an MDIO controller that can
be used to control external PHYs associated with the Industrial
Ethernet peripherals within each ICSSG instance. The MDIO module
used within the ICSSG is similar to the MDIO Controller used
in TI Davinci SoCs. A bus frequency of 1 MHz is chosen for the
MDIO operations.

The nodes are added and enabled in the common k3-j721e-main.dtsi
file by default, and disabled in the existing J721E board dts
file. These nodes need pinctrl lines, and so should be enabled
only on boards where they are actually wired and pinned out for
ICSSG Ethernet. Any new board dts file should disable these if
they are not sure.

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20210601150032.11432-3-s-anna@ti.com