dt-bindings: net: Add Loongson-1 Ethernet Controller

Add devicetree binding document for Loongson-1 Ethernet controller.
And modify MAINTAINERS to add a new F: entry for
Loongson1 dt-binding documents.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>