ARM: dts: am4372: Add PRUSS MDIO controller node

The PRU-ICSS1 instance on AM437x SoCs has a MDIO sub-module that
can be used to control external PHYs associated with the Industrial
Ethernet peripherals within the PRUSS. The MDIO module used within
this PRU-ICSS is an instance of the MDIO Controller used in TI
Davinci SoCs. The same bus frequency of 1 MHz is chosen as the
regular MDIO node. Note that there is no MDIO node added to the
smaller PRU-ICSS0 instance as the MDIO pins are not pinned out.

The node is added and enabled in the common am4372.dtsi file by
default, and disabled in all the existing AM437x board dts files.
This node needs pinctrl lines, and so should be enabled only on
boards where they are actually wired and pinned out for PRUSS
Ethernet. Any new board dts file should disable these if they
are not sure.

Signed-off-by: Andrew F. Davis <afd@ti.com>
[s-anna@ti.com: fix reg address, add commit description]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>