stmmac: pci: Add dwmac support for Loongson

This GMAC module is integrated into the Loongson-2K SoC and the LS7A
bridge chip.

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>