otg: msm_otg: add power_supply usb

To further decouple and step away from using
proprietary APIs between individual driver
have the MSM OTG driver register its own USB
power supply to allow any charger driver to
subscribe to notifications for it and also
use the power_supply_set_online and
power_supply_set_present APIs to notfiy the driver
of USB connections.

For example the 8941 charger PMIC driver handles
the USBIN_VALID interrupt and has to call
power_supply_set_present to notify the OTG driver.

This can now be done from virtually any driver that
provides a supplicant power supply such as "battery"
and provides the external_power_changed callback.

Change-Id: I188d3177f200ab00a7080cd8a361c16728aa0f73
Signed-off-by: David Keitel <dkeitel@codeaurora.org>