dt-bindings: net: Add ICSSG Ethernet

Add a YAML binding document for the ICSSG Programmable real time unit
based Ethernet hardware. The ICSSG driver uses the PRU and PRUSS consumer
APIs to interface the PRUs and load/run the firmware for supporting
ethernet functionality.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>