docs: networking: ti: Add driver doc for AM65 NUSS switch driver

J721e, J7200 and AM64 have multi port switches which can work in multi
mac mode and in switch mode. Add documentation explaining how to use
different modes.

Borrowed from:
Documentation/networking/device_drivers/ethernet/ti/cpsw_switchdev.rst

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>