USB: dwc3-msm: Add sysfs entry to manually set speed

In some cases it is useful to manually force the controller to
run at certain speeds (USB 2.0 or USB 3.0 speeds). For example,
EUD (Emebedded USB Debug) can only work when controller is
operating in USB 2.0 speeds.  User can force USB 2.0 if EUD is
required.

Change-Id: Id022f372014a9218537bd1b5477d4555aff4578c
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>