QUSB2PHY: Fix leakage current issue during wall charger disconnect

It is required to disable QUSB2PHY level shifter by writing to
TCSR_QUSB2PHY_CX2PX_LVL_SHIFT_KEEPER register during disconnect
to avoid leakage current. Hence add support for the same.

Change-Id: I666617d513314903533f2356b5db4b9ca3eac6e4
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>