mtk_clk_register_mux() should be a static function

Fixes: a3ae5499 ("clk: mediatek: Add new clkmux register API")
Signed-off-by: Weiyi Lu <weiyi.lu@mediatek.com>
Link: https://lore.kernel.org/r/1605256192-31307-1-git-send-email-weiyi.lu@mediatek.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>