qcom_dwc3: add initial Qualcomm SoC DWC3 controller glue

This adds some very simple DWC3 glue for the IPQ4018/IPQ4019.
Other chipsets introduce reset line iteration, some further
clock line iteration and some customisations; I'll look at adding
those later.

This is enough to finally bring up USB 3.0 on my IPQ4018 ASUS
RT-58U router.