usb: dwc3: Read snps,bus-suspend-enable property

Driver needs to read this property to enable usb low power
mode in device bus suspend when usb cable is plugged in.

Change-Id: Ie53950541c178b6844af55a1c346d8abeb0b6e7c
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>