net: hns: add attribute reset-field-offset for dsaf node

Add the subctrl reset offset for dsaf, this property is used to reset
xge/ge ports for different dsaf. If this attribute is not present,
default value 0 will be used.

Signed-off-by: Daode Huang <huangdaode@hisilicon.com>
Signed-off-by: Yisen Zhuang <yisen.zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>