phy: stih41x-usb: Remove usb phy driver and dt binding

 documentation.

This phy is only used on STiH415/6 based silicon, and support for
these SoC's is being removed from the kernel.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>