dt: document HiSilicon SPMI controller and mfd/regulator properties

Add documentation for the properties needed by the HiSilicon
6421v600 driver, and by the SPMI controller used to access
the chipset.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/56c40a260bf31edf7049f50586c63ee0d06e02cb.1597762400.git.mchehab+huawei@kernel.org


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