s_cpu_freq_by_mode array was statically initialised with 80MHz CPU
frequency in CPU_MAX and APB_MAX modes, but sdkconfig setting for the
CPU frequency could have been different. For the case of 240MHz CPU
frequency, this would cause a frequency switch between 240MHz and
80MHz to happen, even though such switch is not supported in the fast
path switching functions used by the DFS implementation.
This fixes the issue by moving initialisation into esp_pm_impl_init,
which is called at startup before the first mode switch can happen.
Fixes https://github.com/espressif/esp-idf/issues/1729.
