dt-bindings: arm: Limit cpus schema to only check Arm 'cpu' nodes

Matching on the 'cpus' node was a bad choice because the schema is
incorrectly applied to non-Arm cpus nodes. As we now have a common cpus
schema which checks the general structure, it is also redundant to do so
in the Arm CPU schema.

The downside is one could conceivably mix different architecture's cpu
nodes or have typos in the compatible string. The latter problem pretty
much exists for every schema.
Signed-off-by: Rob Herring <robh@kernel.org>