dt-bindings: riscv: Limit cpus schema to only check RiscV 'cpu' nodes

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

Acked-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Rob Herring <robh@kernel.org>