clk: mediatek: mt2712: Change to use module_platform_driver macro

Now that all of the clocks in clk-mt2712.c are using the common
mtk_clk_simple_{probe,remove}() callbacks we can safely migrate
to module_platform_driver.
While at it, also drop all references to `simple` in the specific
context of mt2712 as that was used in the past only to allow us
to have two platform_driver(s) in one file.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20230306140543.1813621-11-angelogioacchino.delregno@collabora.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>