net: stmmac: Export dwmac1000_dma_ops

Export the DW GMAC DMA-ops descriptor so one could be available in
the low-level platform drivers. It will be utilized to override some
callbacks in order to handle the LS2K2000 GNET device specifics. The
GNET controller support is being 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>