ab8500-fg: A8500 fuel gauge driver

This driver is responsible for provide battery parameters to user space via
sysfs by registers to power supply class. It uses fuel gauge and gpadc driver
in obtaining the battery parameters. These battery properties are used by
abx500 charging algorithm driver to monitor the battery.
Signed-off-by: Arun Murthy <arun.murthy@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>