usb: dwc3: Add a property to disable USB2 LPM

Add an option to disable USB2 LPM from host. There maybe cases where the
user does not want to enable USB2 LPM (e.g. USB2 LPM is broken).
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/devicetree/bindings/usb/dwc3.txt

@@ -37,6 +37,7 @@ Optional properties:
  - phy-names: from the *Generic PHY* bindings; supported names are "usb2-phy"
 	or "usb3-phy".
  - resets: a single pair of phandle and reset specifier
+ - snps,usb2-lpm-disable: indicate if we don't want to enable USB2 HW LPM
  - snps,usb3_lpm_capable: determines if platform is USB3 LPM capable
  - snps,disable_scramble_quirk: true when SW should disable data scrambling.
 	Only really useful for FPGA builds.