clk: mediatek: Add MUX_FLAGS macro

This is required to mark outputs of certain MUXes as CLK_IS_CRITICAL.

Signed-off-by: Jasper Mattsson <jasu@njomotys.info>
Acked-by: Mars Cheng <mars.cheng@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>