staging: hisilicon,hi6421-spmi-pmic.yaml: cleanup descriptions

Minor cleanups at the schema:
 - There's no need to describe interrupt-controller;
 - gpios need a description.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9f421e254061a42b7b598bd96b12f05460c175c9.1624606660.git.mchehab+huawei@kernel.org


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