mfd: Add HI6421 PMIC Core driver

mfd: Add HI6421 PMIC Core driver

This adds driver to support HiSilicon Hi6421 PMIC. Hi6421 includes multi-
functions, such as regulators, codec, ADCs, Coulomb counter, etc.
This driver includes core APIs _only_.

Drivers for individul components, like voltage regulators, are
implemented in corresponding driver directories and files.

Registers in Hi6421 are memory mapped, so using regmap-mmio API.

Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>