SATA: MV: Add support for the optional PHYs

Some Marvell SoCs have a SATA PHY which can be powered off, in order
to save power. Make use of the generic phy framework to control these
phys.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Tejun Heo <tj@kernel.org>