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

commit 177fcdbc165361facb87682ad14ea54d5f38492c from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



commit b9241f54 upstream.

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>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>