mailbox: mtk-cmdq: Dynamically allocate clk_bulk_data structure

Now that the clock probing code uses devm_kasprintf(), there is
no more restriction on the number of GCEs: dynamically allocate
the clk_bulk_data clocks array to improve flexibility and also
to get a slight memory saving on platforms featuring only one
CMDQ mailbox (and consequently only one Global Command Engine).

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