usb: dwc3-msm: Make POWER_SUPPLY_PROP_PRESENT writeable

Allow the usb power_supply's 'present' property to be writeable
from userspace. This will be useful for debugging external VBUS
detection and independently verifying USB device mode functionality.

Change-Id: Ia156cd05f4b5b21243320f2531cbb6613b93b994
Signed-off-by: Jack Pham <jackp@codeaurora.org>