net: hns: add syscon operation for dsaf

This patch provides the read/write function for dsaf to
access the registers through syscon methods.

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>