clk: mediatek: gate: Internalize clk implementation

struct mtk_clk_gate and mtk_clk_register_gate() are not used outside of
the gate clk library. Only the API that handles a list of clks is used
by the individual platform clk drivers.

Internalize the parts that aren't used outside of the implementation.

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/20220208124034.414635-4-wenst@chromium.org


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