net: ethtool: Add generic parts of cable test TDR

Add the generic parts of the code used to trigger a cable test and
return raw TDR data. Any PHY driver which support this must implement
the new driver op.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>

v2
Update nxp-tja11xx for API change.

Signed-off-by: David S. Miller <davem@davemloft.net>