regulator: ab8500: Remove ab8500_regulator_of_probe()

Now this is a DT-only driver because non-devicetree probe path is removed,
so merge ab8500_regulator_of_probe() into ab8500_regulator_probe().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>