dt-bindings: phy: add DT bindings for UniPhier USB2 PHY driver

Add DT bindings for PHY interface built into USB2 controller
implemented on Socionext UniPhier SoCs.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>