dt-bindings: phy: add UniPhier PCIe PHY description

Add DT bindings for PHY interface built into PCIe controller implemented
in UniPhier SoCs.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>