staging: mfd: hi6421-spmi-pmic: Simplify the compatible string

It is clear that this driver is for PMIC. So, get rid of
it at the compatible.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/12bdc9b4734e156098408d0d27256abdc91ac283.1597762400.git.mchehab+huawei@kernel.org


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