mfd: Support HiSilicon Hi6421 PMIC

Add support of HiSilicon Hi6421 in mfd. Hi6421 is a power management and
codec IC from HiSilicon Inc., which includes regulators, codec, ADCs,
Coulomb counter, etc. The way to communitcate with Hi6421 is through
memory mapped I/O ports.

Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>