ARM: dts: imx7s: Define operating points table for cpufreq

Processor operating points for imx7s.dtsi should be properly defined to
perform correct imx-cpufreq-dt probe and registration and provide an
access to the temperature sensors using the i.MX thermal driver.

Signed-off-by: Denys Drozdov <denys.drozdov@toradex.com>
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>