dt-bindings: hisilicon,hi6421-spmi-pmic.yaml: make some rules stricter

Define maxItems for interrupts and accept only "ldo" in lower case.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/b7a775808d9c3a87fbe1c5a6dd71f8f18be7e649.1627116034.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>