net: phy: marvell: Add support for amplitude graph

The Marvell PHYs can measure the amplitude of the returned signal for
a given distance. Implement this option of the cable test
infrastructure. When reporting the step, convert the distance into cm.

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

v2:
Step based on the measurement resolution, and convert this to cm.

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