net: phy: bcm54140: add cable diagnostics support

Use the generic cable tester functions from bcm-phy-lib to add cable
tester support.

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 B, result: Open Circuit
  Pair: Pair C, result: Open Circuit
  Pair: Pair D, result: Open Circuit
  Pair: Pair A, fault length: 106.60m
  Pair: Pair B, fault length: 103.32m
  Pair: Pair C, fault length: 104.96m
  Pair: Pair D, fault length: 106.60m

1m cable, A/B connected, pair 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 D, result: Open Circuit
  Pair: Pair C, fault length: 0.82m
  Pair: Pair D, fault length: 1.64m

1m cable, A/B connected, pair C shorted with D:
  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 to another pair
  Pair: Pair D, result: Short to another pair
  Pair: Pair C, fault length: 1.64m
  Pair: Pair D, fault length: 1.64m

The granularity of the length measurement seems to be 82cm.

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