USB: msm_otg: Allow low power mode with proprietary charger connected

When a proprietary (non-standard) charger is connected, enter low power
mode.  There is no requirement to pull DP to VDP_SRC.  Hence allow PHY
retention and power collapse. Enable PHY asynchronous interrupt for all
cases when PHY comparators are used.

CRs-Fixed: 482226
Change-Id: I54976cabceaa06737e1e5a22313910a3db79ab26
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>