net: hix5hd2_gmac: add tx scatter-gather feature

"hisi-gemac-v2" adds the SG/TXCSUM/TSO/UFO features.
This patch only adds the SG(scatter-gather) driver for transmitting,
the drivers of other features will be submitted later.

Signed-off-by: Dongpo Li <lidongpo@hisilicon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>