net: stmmac: dwmac-loongson: Add phy_interface for Loongson GMAC

PHY-interface of the Loongson GMAC device is RGMII with no internal
delays added to the data lines signal. So to comply with that let's
pre-initialize the platform-data field with the respective enum
constant.

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>