mailbox: mtk-cmdq: Rename gce_plat variable with SoC name postfix

Rename gce_plat variable postfix from 'v1~v7' to SoC names.

Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>