DT doc: net: cpsw mac-address is optional

mac-address is an optional property. If no mac-address is set, a random
mac-address will be generated.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: David S. Miller <davem@davemloft.net>