clk: mediatek: mt8516: Move apmixedsys clock driver to its own file

clk: mediatek: mt8516: Move apmixedsys clock driver to its own file

In preparation for migrating mt8516 clocks to the common simple
probe mechanism, convert the apmixedsys to be a separated
platform driver and move it to clk-mt8516-apmixedsys.c.
While at it, also fix some indentation issues.

During the conversion, error handling was added to the apmixedsys
probe function.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20230306140543.1813621-29-angelogioacchino.delregno@collabora.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>