net: hns: add attribute port-idx-in-ae in enet node.

This patch parse port-idx-in-ae in enet node. In NIC mode of DSAF, all 6
PHYs of service DSAF are taken as ethernet ports to the CPU. The
port-idx-in-ae can be 0 to 5. Here is the diagram:
            +-----+---------------+
            |            CPU      |
            +-+-+-+---+-+-+-+-+-+-+
              |    |   | | | | | |
           debug debug   service
           port  port     port
           (0)   (0)     (0-5)

In Switch mode of DSAF, all 6 PHYs of service DSAF are taken as physical
ports connect to a LAN Switch while the CPU side assume itself have one
single NIC connect to this switch. In this case, the port-idx-in-ae will
be 0 only.
            +-----+-----+------+------+
            |                CPU      |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+
              |    |     service| port(0)
            debug debug  +------------+
            port  port   |   switch   |
            (0)   (0)    +-+-+-+-+-+-++
                          | | | | | |
                         external port

when port-idx-in-ae is not exists, old attribute port-id will be used
(only for compatible purpose, not recommended to use port-id in new code).

Signed-off-by: Daode Huang <huangdaode@hisilicon.com>
Signed-off-by: Yisen Zhuang <yisen.zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>