Add support for bq27521 battery monitor

This adds basic support for BQ27521 battery monitor, used in Nokia N9
and N950. In particular, battery voltage is important to be able to
tell when the battery is almost empty. Emptying battery on N950 is
pretty painful, as flasher needs to be used to recover phone in such case.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Andrew F. Davis <afd@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>