net: ag71xx: add flow control support

Add flow control support. The functionality was tested on AR9331 SoC and
confirmed by iperf3 results and HW counters exported over ethtool.
Following test configurations was used:

iMX6S receiver <--- TL-SG1005D switch <---- AR9331 sender

The switch is supporting symmytric flow control:
Settings for eth0:
        Supported ports: [ MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
--->>   Link partner advertised pause frame use: Symmetric
        Link partner advertised auto-negotiation: Yes
        Link partner advertised FEC modes: Not reported
        Speed: 100Mb/s
        Duplex: Full
        Auto-negotiation: on
        Port: MII
        PHYAD: 4
        Transceiver: external
        Link detected: yes

The iMX6S system was configured to 10Mbit, to let the switch use flow
control:
  - ethtool -s eth0 speed 10

With flow control disabled on AR9331:
  - ethtool -A eth0  rx off tx off
  - iperf3 -u -c 172.17.0.1 -b100M -l1472 -t10

[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec  66.2 MBytes  55.5 Mbits/sec  0.000 ms  0/47155 (0%)  sender
[  5]   0.00-10.04  sec  11.5 MBytes  9.57 Mbits/sec  1.309 ms  38986/47146 (83%)  receiver

With flow control enabled on AR9331:
  - ethtool -A eth0  rx on tx on
  - iperf3 -u -c 172.17.0.1 -b100M -l1472 -t10

[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec  15.1 MBytes  12.6 Mbits/sec  0.000 ms  0/10727 (0%)  sender
[  5]   0.00-10.05  sec  11.5 MBytes  9.57 Mbits/sec  1.371 ms  2525/10689 (24%)  receiver

Similar results are get in opposite direction by introducing extra CPU
load on AR9331:
  - chrt 40 dd if=/dev/zero of=/dev/null &

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>