ARM: dts: imx53-qsb: disable 1.2GHz OPP

The maximum CPU frequency for the i.MX53 QSB is 1GHz, so disable the
1.2GHz OPP. This makes the board work again with configs that have
cpufreq enabled like imx_v6_v7_defconfig on which the board stopped
working with the addition of cpufreq-dt support.

Fixes: 791f4166 ("ARM: dts: imx53: add cpufreq-dt support")

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>