MAINTAINERS: Add entry for Socionext ethernet driver

Add entry for the Socionext Netsec controller driver and DT bindings.

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