devlink: add documentation for hns3 driver

Add a file to document devlink support for hns3 driver, now support devlink
info and devlink reload.

Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>