dt-bindings: interrupt-controller: Convert ARM GIC to json-schema

Convert the ARM GIC binding document to DT schema format using
json-schema.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>