Documentation: usb: fix wrong documentation paths

Fixes wrong spelled "pinctrl-bindings.txt" and "qcom-dwc3-usb-phy.txt"
file names as also wrong specified "mt8173-mtu3.txt" file name.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Rob Herring <robh@kernel.org>