clk: imx51-imx53: Annotate critical clocks as CLK_IS_CRITICAL

Instead of explicitly enabling critical clocks via clk_prepare_enable(),
let's use the standard CLK_IS_CRITICAL flag instead, which makes the code
a bit shorter.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>