clk: mediatek: mux: Internalize struct mtk_clk_mux

struct mtk_clk_mux is an implementation detail of the mux clk type,
and is not used outside of the implementation.

Internalize the definition to minimize leakage of details and shrink
the header file.

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>
Link: https://lore.kernel.org/r/20220208124034.414635-11-wenst@chromium.org


Reviewed-by: Chun-Jie Chen <chun-jie.chen@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>