Add LTC2941/LTC2943 Battery Gauge Driver

Add LTC2941/LTC2943 Battery Gauge Driver

Both the LTC2941 and LTC2943 measure battery capacity.
The LTC2943 is compatible with the LTC2941, it adds voltage and
temperature monitoring, and uses a slightly different conversion
formula for the charge counter.

To avoid confusion with e.g. the LTC2945, the driver is called
LTC2941 instead of LTC294X.

v2: Fix units of measurement: uV, uA and centidegrees.
v3: Correctly set configuration register. Allow negative values
    for the sense resistor.
v4: Run checkpatch.pl and fix all errors and warnings.
v5: Prefix "lltc," to devicetree properties.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
[ removed .owner field ]
Signed-off-by: Sebastian Reichel <sre@kernel.org>