net: phy: realtek: Add RTL8224 cable testing support

net: phy: realtek: Add RTL8224 cable testing support

The RTL8224 can detect open pairs and short types (in same pair or some
other pair). The distance to this problem can be estimated. This is done
for each of the 4 pairs separately.

It is not meant to be run while there is an active link partner because
this interferes with the active test pulses.

Output with open 50 m cable:

  Pair A code Open Circuit, source: TDR
  Pair A, fault length: 51.79m, source: TDR
  Pair B code Open Circuit, source: TDR
  Pair B, fault length: 51.28m, source: TDR
  Pair C code Open Circuit, source: TDR
  Pair C, fault length: 50.46m, source: TDR
  Pair D code Open Circuit, source: TDR
  Pair D, fault length: 51.12m, source: TDR

Terminated cable:

  Pair A code OK, source: TDR
  Pair B code OK, source: TDR
  Pair C code OK, source: TDR
  Pair D code OK, source: TDR

Shorted cable (both short types are at roughly the same distance)

  Pair A code Short to another pair, source: TDR
  Pair A, fault length: 2.35m, source: TDR
  Pair B code Short to another pair, source: TDR
  Pair B, fault length: 2.15m, source: TDR
  Pair C code OK, source: TDR
  Pair D code Short within Pair, source: TDR
  Pair D, fault length: 1.94m, source: TDR

Signed-off-by: Issam Hamdi <ih@simonwunderlich.de>
Co-developed-by: Sven Eckelmann <se@simonwunderlich.de>
Signed-off-by: Sven Eckelmann <se@simonwunderlich.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20251024-rtl8224-cable-test-v1-1-e3cda89ac98f@simonwunderlich.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>