regulator: core: Add set/get_current_limit helpers for regmap users

By setting curr_table, n_current_limits, csel_reg and csel_mask, the
regmap users can use regulator_set_current_limit_regmap and
regulator_get_current_limit_regmap for set/get_current_limit callbacks.

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