dt-bindings: arm: Correct links to idle states definitions

The arm,idle-state DT bindings recently got converted to the json-schema,
but some links are still pointing to the old, non-existing, txt file. Let's
update the links to fix this.

Fixes: baac82fe ("dt-bindings: arm: Convert arm,idle-state binding to DT schema")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>