mfd: hi6421-spmi-pmic: move driver from staging

This driver is ready for mainstream. So, move it out of staging.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/dd150f3ffa19c2dda0171f7dbe1dd63cce2a7af5.1627025657.git.mchehab+huawei@kernel.org


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