regulator: qcom_spmi: Use regmap helpers for enable/disable/is_enabled callback

Setup .enable_reg/.enable_mask/.enable_val fields, then we can use the
regmap helpers for enable/disable/is_enabled callback implementation.

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