29.6.2.1.3 MCKn Clock Frequency
When the I2S is in Host mode, writing a '1' to CLKCTRLn.MCKEN will output GCLK_I2S_n as Host Clock to the MCKn pin. The Host Clock to MCKn pin can be divided by writing to CLKCTRLn.MCKSEL and CLKCTRLn.MCKOUTDIV. The Host Clock (MCKn) frequency is GCLK_I2S_n frequency divided by (MCLKOUTDIV+1).