USB: dwc3-msm: Add support for setting specific frequency for core clock

Add support for setting USB core clock to particular frequency so that
core clock frequency can be passed through dts property.

Change-Id: If9ff41037d22d7be7f09c9468e8d4cc92280a28e
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>