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

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

Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>