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@xxxxxxxxxxxx>
Reviewed-by: Miles Chen <miles.chen@xxxxxxxxxxxx>