ARM: dts: nxp/imx: limit sk-imx53 supported frequencies

The SK-IMX53 board, bearing i.MX536A CPU, is not stable when running at
1.2 GHz (default iMX53 maximum). The SoC is only rated up to 800 MHz.
Disable 1.2 GHz and 1 GHz frequencies.

Fixes: 0b8576d8 ("ARM: dts: imx: Add support for SK-iMX53 board")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>