dt-bindings: arm: optee: add interrupt property

Adds an optional interrupt property to the optee binding.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>