regulator: core: Refactor devm_regulator_get* APIs

The implementation of devm_regulator_get, devm_regulator_get_exclusive and
devm_regulator_get_optional are almost the same.
Introduce _devm_regulator_get helper function and refactor the code.

Also move devm_regulator_get_exclusive to proper place, put it after
regulator_get_exclusive() function.

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