net: phy: at803x: add cable diagnostics support

The AR8031/AR8033 and the AR8035 support cable diagnostics. Adding
driver support is straightforward, so lets add it.

The PHY just do one pair at a time, so we have to start the test four
times. The cable_test_get_status() can block and therefore we can just
busy poll the test completion and continue with the next pair until we
are done.
The time delta counter seems to run at 125MHz which just gives us a
resolution of about 82.4cm per tick.

100m cable, A/B/C/D open:
  Cable test started for device eth0.
  Cable test completed for device eth0.
  Pair: Pair A, result: Open Circuit
  Pair: Pair A, fault length: 107.94m
  Pair: Pair B, result: Open Circuit
  Pair: Pair B, fault length: 104.64m
  Pair: Pair C, result: Open Circuit
  Pair: Pair C, fault length: 105.47m
  Pair: Pair D, result: Open Circuit
  Pair: Pair D, fault length: 107.94m

1m cable, A/B connected, C shorted, D open:
  Cable test started for device eth0.
  Cable test completed for device eth0.
  Pair: Pair A, result: OK
  Pair: Pair B, result: OK
  Pair: Pair C, result: Short within Pair
  Pair: Pair C, fault length: 0.82m
  Pair: Pair D, result: Open Circuit
  Pair: Pair D, fault length: 0.82m

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>