usb: core: phy: add the SPDX-License-Identifier and include guard

This clarifies the license of the code. While here also add an include
guard to the header file.

Fixes: 07dbff0d ("usb: core: add a wrapper for the USB PHYs on the HCD")
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>