dt-bindings: mailbox: mediatek,gce-mailbox: Add support for MT6795

Add a compatible string for the MT6795 Helio X10 SoC using MT8173
binding and add a header for the MT6795's GCE mailbox.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>