power: supply: ab8500: Use core battery parser

This deploys the core battery DT parser to read the basic properties
of the battery. We only use very little of it as we start out, but
we will improve as we go along.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>