power: supply: ab8500: Rename charging algorithm symbols

The "abx500" name on the charging algorithm stems from the ambition
to produce a series of these analog basebands, re-using the same
charging algorithm driver. No ASICs beside AB8500 and AB8505 were
ever produced so this terminology is confusing. Rename the
algorithm file and symbols to reflect the more narrow scope.

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