arm: dts: Add support for A72 specific AM68 Starter Kit Base Board

The SK architecture comprises of baseboard and a SOM board. The
AM68 Starter Kit's baseboard contains most of the actual connectors,
power supply etc. The System on Module (SoM) is plugged on to the base
board. Therefore, add support for peripherals brought out in the base
board.

Schematics: https://www.ti.com/lit/zip/SPRR463



Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>