Add I2S MCLK support to iMX RT

[tannewt]

Raises NotImplementedError on all other ports with I2S implementations.

[jepler]

I'm not super familiar with mclk. Does its frequency need to be set, or can it be automatically derived? Wikipedia (not the best source I know) says "Master clock [is] typically 256 x LRCLK" while one random datasheet I found shows it varying over specific values from 128fs to 1152fs (https://www.digikey.com/htmldatasheets/production/755129/0/0/1/ak4430.html)

[tannewt]

I'm not super familiar with mclk. Does its frequency need to be set, or can it be automatically derived? Wikipedia (not the best source I know) says "Master clock [is] typically 256 x LRCLK" while one random datasheet I found shows it varying over specific values from 128fs to 1152fs (https://www.digikey.com/htmldatasheets/production/755129/0/0/1/ak4430.html)

In the iMX case, the audio code derives a reasonable value to generate the bitclock that the WM8960 is ok with. I think it should generally be a clock faster than the bitclock and in the same phase. I omited setting frequency here to leave it vague until we have a specific need. The I2SOut clock frequency depends on the sample rate so it isn't clear how to define mclk. So, for now, I thought I'd omit it.

[jepler]

Thanks for addressing my review questions!