power: supply: Add support for MAX1721x standalone fuel gauge

The MAX17211 monitor a single cell pack. The MAX17215 monitor and
balance a 2S or 3S pack or monitor a multiple-series cell pack.
Both device use 1-Wire interfce.

Signed-off-by: Alex A. Mihaylov <minimumlaw@rambler.ru>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>