net: stmmac: Add multi-channel support

DW GMAC v3.73 can be equipped with the Audio Video (AV) feature which
enables transmission of time-sensitive traffic over bridged local area
networks (DWC Ethernet QoS Product). In that case there can be up to two
additional DMA-channels available with no Tx COE support (unless there is
vendor-specific IP-core alterations). Each channel is implemented as a
separate Control and Status register (CSR) for managing the transmit and
receive functions, descriptor handling, and interrupt handling.

Add the multi-channels DW GMAC controllers support just by making sure the
already implemented DMA-configs are performed on the per-channel basis.

Note the only currently known instance of the multi-channel DW GMAC
IP-core is the LS2K2000 GNET controller, which has been released with the
vendor-specific feature extension of having eight DMA-channels. The device
support will be added in one of the following up commits.

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>