bq2415x_charger: Add shutdown handler.

Reset bq2415x_charger upon shutdown of the host.
msm_battery: Add support for charger valid detection.

Sometimes the target is referred to as charging is VBUS is on.
On these targets, OTG function will show as if the target is charging,
reporting wrong information.

Add support for targets to perform a double check for this behaviour.
u8800: Remove PMIC notifier about VBUS.

Sometimes the charger is not properly detected when this is turned on. Could
be related to the fact that VBUS on is triggered when OTG function is turned on.
u8800: Add support for charger valid detection.