smb347-charger: Convert to regmap API

The smb347-charger driver does a lot of read-modify-write to the device
registers. Instead of open-coding everything we can take advantage of
regmap API which provides nice functions to do this kind of things.

In addition there is no need for custom debugfs file for dumping
registers as this is already provided by the regmap API.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>