usb: gadget: Add support for USB going into LPM on bus suspend

This change adds the ability of the msm_otg to go to LPM when
USB cable is connected and bus is suspended.
This ability is enabled by a board file parameter.

The msm_otg is notified about SUSPEND/RESUME events via the
UDC layer. New event for RESUME was defined for that.

Change-Id: Ic508f1898cc3b57ab76eccd379bea38ed363570b
Signed-off-by: Amit Blay <ablay@codeaurora.org>