staging: mfd: add a PMIC driver for HiSilicon 6421 SPMI version

Add the PMIC SPMI driver for the HiSilicon 6421v600.

[mchehab+huawei@kernel.org: keep just the MFD driver on this patch,
 and renamed filenames to better match other upstream drivers]

 The compete patch is at:
	https://github.com/96boards-hikey/linux/commit/08464419fba2



Signed-off-by: Mayulong <mayulong1@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4ffb2694244baa47387e39e2c5d71243242c1fc1.1597647359.git.mchehab+huawei@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>