This is to resolve issue reported in https://github.com/espressif/esp-idf/issues/130.
1. Modify I2S DAC built-in mode settings 2. Add I2S pdm mode 3. Correct I2S signal index in full-duplex mode 4. Modify dac output function 5. Add dac pad init functions 6. Modify DAC documentation 7. Add i2s_set_dac_mode API