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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>