usb: dwc3: add generic OF glue layer

For simple platforms which merely enable some clocks
and populate its children, we can use this generic
glue layer to avoid boilerplate code duplication.

For now this supports Qcom and Xilinx, but if we
find a way to add generic handling of regulators and
optional PHYs, we can absorb exynos as well.

Tested-by: Subbaraya Sundeep Bhatta <subbaraya.sundeep.bhatta@xilinx.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>