usb: dwc3: Set GUSB2PHYCFG.ENBLSLPM

GUSB2PHYCFG.ENBLSLPM enables the controller to assert low power signals
to the PHY. Unless disabled via device property, explicitly set
GUSB2PHYCFG.ENBLSLPM as it may not be set by default.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>