net: phy: LAN87xx: add ethtool SQI support

This patch add the support for measuring Signal Quality Index for
LAN87xx and LAN937x T1 Phy. It uses the SQI Method 5 for obtaining the
values.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>