regulator: add driver for hi6421 voltage regulator

Add driver support for HiSilicon Hi6421 voltage regulators.

Two rules for regulator enabling are defined in hi6421 spec:
1) Between disable and enable of each regulator (LDOs or BUCKs), there must
   be a protection gap. Use @off_on_delay of regulator core to implement this.
2) No two regulators can be enabled at the same time. Use mutex in
   hi6421_regulator_pdata to ensure this. A protection gap of 100us is added
   into each LDO/BUCK's .enable_time.

Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>