regulator: sc2731: Add regulator driver to support Spreadtrum SC2731 PMIC

Add regulator driver for Spreadtrum SC2731 device.
It has 17 general purpose LDOs, BUCKs generator and
digital output to control regulators.

Signed-off-by: Erick Chen <erick.chen@spreadtrum.com>
Reviewed-by: Baolin Wang <baolin.wang@spreadtrum.com>
Signed-off-by: Mark Brown <broonie@kernel.org>