dac_continuous_config_t::freq_hz field

The frequency of DAC conversion in continuous mode, unit: Hz The supported range is related to the target and the clock source. For the clock `DAC_DIGI_CLK_SRC_DEFAULT`: the range is 19.6 KHz to several MHz on ESP32 and 77 Hz to several MHz on ESP32-S2. For the clock `DAC_DIGI_CLK_SRC_APLL`: the range is 648 Hz to several MHz on ESP32 and 6 Hz to several MHz on ESP32-S2. Typically not suggest to set the frequency higher than 2 MHz, otherwise the severe distortion will appear