Support "RX BCLK" when transmitting audio data

[jepler]

It's possible to use a pin designated "RX BCLK" when transmitting audio data. This can be done, but circuitpython lacks the necessary code. This may be necessary for some boards such as the micromod.

Here's a snippet from micropython:

    } else if ((self->mode == TX) && (i2s_gpio_map[sck_index].mode == RX)) {                 
        saiConfig.syncMode = kSAI_ModeAsync;                                                 
        SAI_RxSetConfig(self->i2s_inst, &saiConfig);                                         
        saiConfig.bitClock.bclkSrcSwap = true;                                               
        saiConfig.syncMode = kSAI_ModeSync;                                                  
        SAI_TxSetConfig(self->i2s_inst, &saiConfig);