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>