power: supply: ab8500: Use only one battery type

The code was going through hoops and loops to detect what
battery is connected and check the resistance for this battery
etc.

Skip this trouble: we will support one battery (currently
"unknown") then we will find the connected battery in the
device tree using a compatible string. The battery resistance
may be used to double-check that the right battery is
connected.

Convert the array of battery types into one battery type so
we can next move over the properties of this one type into
the standard struct power_supply_battery_info.

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