cpufreq: Add i.MX7S to cpufreq-dt-platdev blocklist

The i.MX 7Solo currently does not have multiple operating points,
however, in order for the i.MX Thermal driver to successfully probe
a cpufreq device is required. Add it to the cpufreq-dt-platdev
driver's blocklist to allow using imx-cpufreq-dt.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Cc: Stefan Agner <stefan@agner.ch>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>