power: supply: add CellWise cw2015 fuel gauge driver

This patch adds a driver for the CellWise cw2015 fuel gauge.

The CellWise cw2015 is a shuntless, single-cell Li-Ion fuel gauge used
in the pine64 Pinebook Pro laptop and some Raspberry Pi UPS HATs.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>