Phy: DT binding documentation for Marvell MVEBU SATA phy.

Describe the binding for the Marvell MVEBU SATA phy. This driver
can be used at least with Kirkwood, Dove and maybe others.
Additionally, update the SATA binding with the properties to link
to the phy nodes.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>