USB: EHCI: Vote for VDD min only when Liquid is undocked

Currently, ehci driver votes for VDD minimization when USB bus goes
to suspend. PHY will not generate interrupt if driver votes for VDD
minimization. Hence set the VDDCX suspend voltage to 0.75V when Liquid
is attached to docking station, so that phy can generate interrupt
as a result of a device plug in.

Change-Id: Iab0bc2855e7729f30ca5665014670e8fa6ca313d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>