net: stmmac: dwmac-loongson: Init ref and PTP clocks rate

Reference and PTP clocks rate of the Loongson GMAC devices is 125MHz.
(So is in the GNET devices which support is about to be added.) Set
the respective plat_stmmacenet_data field up in accordance with that
so to have the coalesce command and timestamping work correctly.

Fixes: 30bba69d ("stmmac: pci: Add dwmac support for Loongson")
Signed-off-by: Feiyang Chen <chenfeiyang@loongson.cn>
Signed-off-by: Yinggang Gu <guyinggang@loongson.cn>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>