clk: mediatek: clk-mtk: Register MFG notifier in mtk_clk_simple_probe()

In preparation for commonizing topckgen probe on various MediaTek SoCs
clock drivers, add the ability to register the MFG MUX notifier in
mtk_clk_simple_probe() by passing a custom notifier register function
pointer, as this function will be slightly different across different
SoCs.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Miles Chen <miles.chen@mediatek.com>
Link: https://lore.kernel.org/r/20230120092053.182923-19-angelogioacchino.delregno@collabora.com
Tested-by: Mingming Su <mingming.su@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>