arm64: dts: mediatek: mt8188: Add Global Command Engine mailboxes

In preparation for adding multimedia nodes and power domains, add
support for the two Global Command Engine (GCE) mailboxes found in
this SoC.

Link: https://lore.kernel.org/r/20240527093908.97574-3-angelogioacchino.delregno@collabora.com


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>