usb: ehci: vote for lower vddcx voltage during low power mode

usb phy requires vddcx to be at 1.1V when operational. However,
keeping vddcx high all times will cause higher power consumption.
Hence, request for lower vddcx voltage during low power mode.

Change-Id: I13465a65770b91178c82b722de9fb5fd132fe966
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>