dt-bindings: net: vsc8531: add two additional LED modes for VSC8584

The VSC8584 (and most likely other PHYs in the same generation) has two
additional LED modes that can be picked, so let's add them.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>