USB: EHCI: Add support to detect docking station connection and removal

This change implements the docking station detection by reading the status
of MPP9 on PMIC8921. With this change VREG_5V(USB port 3 VBUS) is turned on
only when docking station is connected to Liquid platform in order to save
power.

Change-Id: I4282b4273adabf8aeaf8faad43cd2cfff36eeac6
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>