staging: hikey9xx: hisilicon, hi6421-spmi-pmic.yaml: cleanup a warning

There's no additionalProperties field at the yaml file, causing
a warning when checking it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1920935fc7320f8d03ed3c89625fa865adcf4390.1611212783.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/hikey9xx/hisilicon,hi6421-spmi-pmic.yaml

@@ -60,6 +60,8 @@ required:
   - reg
   - regulators
 
+additionalProperties: false
+
 examples:
   - |
     /* pmic properties */