samples: i2s: echo: Enable nRF53 MCK output in I2S sample

Enable the MCK output to a pin to allow for DAC and ADC clocking.
Provides a MCK source to peripherals that can utilize an external
clocking source for internal DSP processes, especially at higher
MCK/LRCK ratios.

Signed-off-by: Sean O'Connor <[email protected]>

Author: standalone-tech

samples/drivers/i2s/echo/boards/nrf5340dk_nrf5340_cpuapp.overlay

@@ -22,7 +22,8 @@
 
 	i2s0_default_alt: i2s0_default_alt {
 		group1 {
-			psels = <NRF_PSEL(I2S_SCK_M, 1, 15)>,
+			psels = <NRF_PSEL(I2S_MCK, 0, 12)>,
+				<NRF_PSEL(I2S_SCK_M, 1, 15)>,
 				<NRF_PSEL(I2S_LRCK_M, 1, 12)>,
 				<NRF_PSEL(I2S_SDOUT, 1, 13)>,
 				<NRF_PSEL(I2S_SDIN, 1, 14)>;