dt-bindings: net: phy: mscc: vsc8531: factorize vsc8531, led-N-mode

VSC8584 supports 4 LEDs while VSC8531 only supports 2. Let's factorize
the documentation for LED mode properties and give the 4 default values
(the first two being shared between VSC8531 and VSC8584).

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>