net: phy: marvell: Implement led_blink_set()

The Marvell PHY can blink the LEDs, simple on/off. All LEDs blink at
the same rate, and the reset default is 84ms per blink, which is
around 12Hz.

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>