mailbox: mtk-cmdq: Stop requiring name for GCE clock

The Global Command Engine mailbox has only one clock hence
requiring clock-names is useless.

Get the first (and only) clock instead, without name checks.

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