net: stmmac: add a parse for new property 'snps,clk-csr'

Parse new property 'snps,clk-csr' firstly because the new property
is documented in binding file, if failed, fall back to old property
'clk_csr' for legacy case

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