net: hns3: add phy selftest function

Currently, the loopback test supports only mac selftest and serdes
selftest. This patch adds phy selftest.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>