dt-bindings: mailbox: mediatek: Add support for MT8196 GCE mailbox

Add the compatible name and iommus property for MT8196.

In MT8196, all command buffers allocated and used by the GCE device
work with IOMMU.

Signed-off-by: Jason-JH Lin <jason-jh.lin@mediatek.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>