devlink: add documentation for hns3 driver

mainline inclusion
from mainline-master
commit 6149ab60
category: feature
bugzilla: 173966
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6149ab604c80a20e5741bea6c90583edde15c488



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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>
Reviewed-by: Yongxin Li <liyongxin1@huawei.com>
Signed-off-by: Junxin Chen <chenjunxin1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>