clk: mediatek: mtk: Clean up included headers

Some included headers aren't actually used anywhere, while other headers
with the declaration of functions and structures aren't directly
included.

Get rid of the unused ones, and add the ones that should be included
directly.

On the header side, replace headers that are included purely for data
structure definitions with forward declarations. This decreases the
amount of preprocessing and compilation effort required for each
inclusion.

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/20220208124034.414635-21-wenst@chromium.org


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