arm64: dts: HiSilicon: Add usb mux hub for HiKey 960

Add dt bindings for HiKey 960 USB HUB. Such board comes with an
integrated USB HUB provided via a Microchip USB5734 4-port high-speed
hub controller.

[mchehab: modified it to adapt to the merged DT schema]

Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>