dt-bindings: mtk-cirq: Add binding document

This commit adds the device tree binding document for
the mediatek cirq.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Youlin Pei <youlin.pei@mediatek.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>