ARM: dts: keystone-k2g: Add PRUSS MDIO controller nodes

The PRUSSs on 66AK2G SoCs contain an MDIO controller that can
be used to control external PHYs associated with the Industrial
Ethernet peripherals within each 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 2.5 MHz is chosen
as the regular MDIO node.

The nodes are added to the common keystone-k2g dtsi file and are
disabled. These need to be enabled in the respective board files
supporting 66AK2G SoCs and where the ethernet is pinned out and
connected properly.

Signed-off-by: Suman Anna <s-anna@ti.com>