power: supply: max17040: Support setting rcomp

The Maxim ModelGauge family supports fine-tuning by setting
a compensation value (named rcomp in the docs). The value is affected by
battery chemistry and ambient temperature.

Add support for reading maxim,rcomp from DT and configuring the device
with the supplied value. Temperature adjustment is not implemented at
the moment, because there is no provision for receiving the ambient
temperature at the moment.

Signed-off-by: Iskren Chernev <iskren.chernev@gmail.com>
Tested-by: Jonathan Bakker <xc-racer2@live.ca>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>