RDMA/hns: Add mailbox's implementation for hip08 RoCE driver

In hip08 SoC, the hardware implementation of mailbox command
has changed with hip06 SoC. As a result, it adjusts the
architecture of the command code and implements the interfaces
of mailbox for hip08 SoC.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Shaobo Xu <xushaobo2@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>