regulator: hi6421v600-regulator: move it from staging

This driver is ready for mainstream. Move it out of staging.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/815b79a4e93f133478d9a5b2dd429526dcfe1dde.1616695231.git.mchehab+huawei@kernel.org


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