ARM: dts: am33xx-l4: Add PRUSS MDIO controller node

The PRUSS on AM335x 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 the
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.

The node is added to the common am33xx-l4.dtsi file and is disabled.
This needs to be enabled in the respective board files using the
relevant AM335x SoCs supporting PRUSS and where the ethernet is
pinned out and connected properly.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>