net: document SMII and correct phylink's new validation mechanism

SMII has not been documented in the kernel, but information on this PHY
interface mode has been recently found. Document it, and correct the
recently introduced phylink handling for this interface mode.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/E1mmfVl-0075nP-14@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>