usb: dwc3: core: Disable internal clock gating conditionally

Currently USB DWC3 controller's internal clock gating is disabled
unconditionally. In few platform, it is possible to enable internal
clock gating with controller. Hence this change adds support to
disable this functionality conditionally using "snps,disable-clk-gating"
device tree property. With this change USB controller's internal clock
gating is enabled by default.

Change-Id: I17d43a23d3bff0cb516b952c35c4a13af53f7777
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>