clk: mediatek: cpumux: Clean up included headers

Some headers with the declaration of functions and structures aren't
directly included. Explicitly include them so that future changes to
other headers would not result in an unexpected build break.

On the header side, add forward declarations for any data structures
whose pointers are used in function signatures. No headers are
required.

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/20220208124034.414635-9-wenst@chromium.org


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