dt-bindings: net: phy: Document LED inactive high impedance mode

Document LED inactive high impedance mode to set the LED to require high
impedance configuration to be turned OFF.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Lee Jones <lee@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240125203702.4552-3-ansuelsmth@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>