dts: hi3660: Add support for basic usb gadget on Hikey960

This patch adds basic core dwc3, usb phy and rt1711h nodes for
usb support on Hikey960.

This does not enable the mux/hub functionality on the board, so
the USB-A host ports will not function, but does allow the USB-C
port to function in gadget mode (unfortunately not in host, as
the hub/mux functionality is needed to enable vbus output to
power devices in host mode).

This is based on an old patch originally by Yu Chen.

Cc: Yu Chen <chenyu56@huawei.com>
Cc: Chunfeng Yun <chunfeng.yun@mediatek.com>
Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: Binghui Wang <wangbinghui@hisilicon.com>
Cc: YongQin Liu <yongqin.liu@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: devicetree@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>