dt-bindings: net: snps,dwmac: add new property snps,clk-csr

Add description for new property snps,clk-csr in binding file

Signed-off-by: Jianguo Zhang <jianguo.zhang@mediatek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>