clk: mediatek: mt7622: Move apmixedsys clock driver to its own file

clk: mediatek: mt7622: Move apmixedsys clock driver to its own file

In preparation for migrating mt7622 clocks to the common simple
probe mechanism, move apmixedsys clocks to a different file.
While at it, use the builtin_platform_driver() macro for it.

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/20230306140543.1813621-25-angelogioacchino.delregno@collabora.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>