clk: mediatek: pll: 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.

Also, copy the MHZ macro from clk-mtk.h, and drop clk-mtk.h from the
included headers.

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/20220208124034.414635-15-wenst@chromium.org


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