arm64: dts: mediatek: Add GCE header for MT8196

Add GCE header define for GCE Thread Priority and GCE Event IDs
that used in the MT8196 dtsi.

Signed-off-by: Jason-JH Lin <jason-jh.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>