dt-bindings: altera: Convert clkmgr binding to json-schema

Convert Altera clkmgr to DT schema format using json-schema.

Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Dinh Nguyen <dinguyen@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>