arm64: dts: mt8183: change rpmsg property name

The the rpmsg property name is changed to "mediatek," to sync with the
vendor name defined in vendor-prefixes.yaml.

Signed-off-by: Tinghan Shen <tinghan.shen@mediatek.com>
Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20210924033935.2127-7-tinghan.shen@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>