staging: regulator: add a regulator driver for HiSilicon 6421v600 SPMI PMIC

Add the regulator driver for the LDO lines provided by the
HiSilicon 6421v600 SPMI PMIC device.

[mchehab+huawei@kernel.org: keep just the regulator driver on this patch,
 renaming it to better fit at upstream namespace]

The compete patch is at:
	https://github.com/96boards-hikey/linux/commit/08464419fba2Signed-off-by: Mayulong <mayulong1@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6e5f6a811edf77575ddaa84ab6542cc646024423.1597647359.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>