Documentation: devicetree: net: add NS2 bindings to amac

Clean-up the documentation to the bgmac-amac driver, per suggestion by
Rob Herring, and add details for NS2 support.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>