net: phy: marvell: Add cable test support

The Marvell PHYs have a couple of different register sets for
performing cable tests. Page 7 provides the simplest to use.

v3:
s/mavell/marvell/g
Remove include of <uapi/linux/ethtool_netlink.h>

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>