net: stmmac: dwmac-loongson: Add Loongson GNET support

The new generation Loongson LS2K2000 SoC and LS7A2000 chipset are
equipped with the network controllers called Loongson GNET. It's the
single and multi DMA-channels Loongson GMAC but with a PHY attached.
Here is the summary of the DW GMAC features the controller has:

   DW GMAC IP-core: v3.73a
   Speeds: 10/100/1000Mbps
   Duplex: Full (both versions), Half (LS2K2000 GNET only)
   DMA-descriptors type: enhanced
   L3/L4 filters availability: Y
   VLAN hash table filter: Y
   PHY-interface: GMII (PHY is integrated into the chips)
   Remote Wake-up support: Y
   Mac Management Counters (MMC): Y
   Number of additional MAC addresses: 5
   MAC Hash-based filter: Y
   Hash Table Size: 256
   AV feature: Y (LS2K2000 GNET only)
   DMA channels: 8 (LS2K2000 GNET), 1 (LS7A2000 GNET)

Let's update the Loongson DWMAC driver to supporting the new Loongson
GNET controller. The change is mainly trivial: the driver shall be
bound to the PCIe device with DID 0x7a13, and the device-specific
setup() method shall be called for it. The only peculiarity concerns
the integrated PHY speed change procedure. The PHY has a weird problem
with switching from the low speeds to 1000Mbps mode. The speedup
procedure requires the PHY-link re-negotiation. So the suggested
change provide the device-specific fix_mac_speed() method to overcome
the problem.

Signed-off-by: Feiyang Chen <chenfeiyang@loongson.cn>
Signed-off-by: Yinggang Gu <guyinggang@loongson.cn>
Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>