clk: imx: Add CLK_IS_CRITICAL flag for busy divider and busy mux

The busy divider and busy mux is actually used by the system critical clocks,
so add 'CLK_IS_CRITICAL' to clocks registered with these two type.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>