ethtool: provide UAPI for PHY Signal Quality Index (SQI)

Signal Quality Index is a mandatory value required by "OPEN Alliance
SIG" for the 100Base-T1 PHYs [1]. This indicator can be used for cable
integrity diagnostic and investigating other noise sources and
implement by at least two vendors: NXP[2] and TI[3].

[1] http://www.opensig.org/download/document/218/Advanced_PHY_features_for_automotive_Ethernet_V1.0.pdf
[2] https://www.nxp.com/docs/en/data-sheet/TJA1100.pdf
[3] https://www.ti.com/product/DP83TC811R-Q1

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>