dt-bindings: net: Add DT bindings for Socionext Netsec

This patch adds documentation for Device-Tree bindings for the
Socionext NetSec Controller driver.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>