dt-bindings: phy: Add PHY_TYPE_USXGMII definition

Add definition for USXGMII phy type.

Signed-off-by: Swapnil Jakhade <sjakhade@cadence.com>
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>