clk: mediatek: mt2712-apmixedsys: Add .remove() callback for module build

Add a .remove() callback to the apmixedsys driver to allow full module
build; while at it, also change the usage of builtin_platform_driver()
to module_platform_driver() to actually make use of the new callback.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20230306140543.1813621-10-angelogioacchino.delregno@collabora.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>