net: ethtool: Add attributes for cable test reports

Add the attributes needed to report cable test results to userspace.
The reports are expected to be per twisted pair. A nested property per
pair can report the result of the cable test. A nested property can
also report the length of the cable to any fault.

v2:
Grammar fixes
Change length from u16 to u32
s/DEV/HEADER/g
Add status attributes
Rename pairs from numbers to letters.

v3:
Fixed example in document
Add ETHTOOL_A_CABLE_NEST_* enum
Add ETHTOOL_MSG_CABLE_TEST_NTF to documentation

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