dt-bindings: arm: Convert cpu binding to json-schema

Convert ARM CPU binding to DT schema format using json-schema.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: devicetree@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>