net: phy: marvell10g: add energy detect power down tunable

Add support for the energy detect power down tunable, which saves
around 600mW when the link is down. The 88x3310 supports off, rx-only
and NLP every second. Enable EDPD by default for 88x3310.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>