net: hisilicon: Add group field to adapt HI13X1_GMAC

In general, group is the same as the port, but some
boards specify a special group for better load
balancing of each processing unit.

Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>