dt-bindings: phy: Add support for HiSilicon's hi3660 USB PHY

This patch adds binding documentation for supporting the hi3660 usb
phy on boards like the HiKey960.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Binghui Wang <wangbinghui@hisilicon.com>
Signed-off-by: Yu Chen <chenyu56@huawei.com>