dt-bindings: mailbox: mediatek,gce-mailbox: Make clock-names optional

The GCE Mailbox needs only one clock and the clock-names can be
used only by the driver (which, for instance, does not use it),
and this is true for all of the currently supported MediaTek SoCs.

Stop requiring to specify clock-names on all non-MT8195 GCEs.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>