power_supply: Add support for tps65217-charger.

This patch adds support for the tps65217 charger driver. This driver is
responsible for controlling the charger aspect of the tps65217 mfd.
Currently, this mainly consists of turning on and off the charger, but
some other features of the charger can be supported through this driver.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>