net: hns3: Add mailbox support to PF driver

Command queue provides the provision of Mailbox command which
can be used for communication between PF and VF. PF handles
messages from various VFs for fetching various information like,
queue, vlan, link status related etc. It also handles the request
from various VFs to perform certain privileged operations.

This patch adds the support of a message handler for handling
such various command requests from VF.

Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: lipeng <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>