dt-bindings: phy: mvebu-utmi: add UTMI PHY bindings

Add bindings for Marvell Armada 3700 USB2 UTMI+ PHY.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>