net: stmmac: dwmac-loongson: Introduce PCI device info data

The Loongson GNET device support is about to be added in one of the
next commits. As another preparation for that introduce the PCI device
info data with a setup() callback performing the device-specific
platform data initializations. Currently it is utilized for the
already supported Loongson GMAC device only.

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>