clk: mediatek: Make mtk_clk_register_composite() static

mtk_clk_register_composite() is not used anywhere outside of the file it
is defined.

Make it static.

Fixes: 9741b1a6 ("clk: mediatek: Add initial common clock support for Mediatek SoCs.")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Miles Chen <miles.chen@mediatek.com>
Link: https://lore.kernel.org/r/20220519071610.423372-2-wenst@chromium.org


Signed-off-by: Stephen Boyd <sboyd@kernel.org>