MSM: Add USB suport for QSD8x50

OTG driver takes care of putting hardware into low power mode.  Hence
make peripheral and host devices as children of OTG device and let
runtime PM takes care of notifying peripheral and host state to
OTG device.  VBUS power up and shutdown routines are implemented by
modem processor.  As RPC infrastructure is not available, configure
USB in peripheral only mode.

Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>