dwc3: Add support to disable clock gating with USB controller

In some cases it is suggested to disable clock gating functionality of
USB controller. This change adds required support for same.

Change-Id: I6e99ce1f823f246153a26706e6209589e9142f67
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>