dt-binding: ti: am65x: document mcu cpsw nuss

Document device tree bindings for The TI AM654x/J721E SoC Gigabit Ethernet MAC
(Media Access Controller - CPSW2G NUSS). The CPSW NUSS provides Ethernet packet
communication for the device.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Murali Karicheri <m-karicheri2@ti.com>
Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>