net: phy: marvell10g: implement suspend/resume callbacks

This patch adds the suspend/resume callbacks for Marvell 10G PHYs. The
three PCS (base-t, base-r and 1000base-x) are set in low power (the PCS
are powered down) when the PHY isn't used.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>