usb: dwc3: Update core clock rate based on USB port speed.

Core clock rate can be reduced or increased based on operating
speeds. Controller starts in Super Speed mode (higher core clock
rate) and it will operate in super or high or full or low speed
upon device connection. Update the core clock rate based on bus
speed to allows system to operate in better low power state (such
as SVS1/SVS2 based on system configuration). High Speed rate for
core clock is programmed from dtsi. Super Speed rate will be used
if High speed core clock rate is not provided for backward
compatibility.

Change-Id: I265149d34de19ab50bd7f106a670a7112bfae384
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>