MAINTAINERS: add an entry for HiSilicon 6421v600 drivers

Add an entry for the SPMI, MFD and PMIC parts of the
HiSilicon 6421v600 support.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/14b009ef0255d61eeaf4273e9c36dafdb1e5e12f.1597762400.git.mchehab+huawei@kernel.org


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