staging: mfd: hi6421-spmi-pmic: add it to the building system

Now that the driver is ready, place it at the build system.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/952e11ea1a33beaf67d6dc355d5c0f99ab4fb964.1597647359.git.mchehab+huawei@kernel.org


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