i2s_std_clk_config_t struct

I2S sample rate.

Choose clock source, see `soc_periph_i2s_clk_src_t` for the supported clock sources. selected `I2S_CLK_SRC_EXTERNAL` (if supports) to enable the external source clock input via MCLK pin,.

The multiple of MCLK to the sample rate Default is 256 in the helper macro, it can satisfy most of cases, but please set this field a multiple of `3` (like 384) when using 24-bit data width, otherwise the sample rate might be inaccurate.