net: phy: marvell: Add software control of the LEDs

Add a brightness function, so the LEDs can be controlled from
software using the standard Linux LED infrastructure.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>