drivers: i2s_mcux_sai: control MCLK direction with DT property

For the SAI peripheral, the MCLK signal input/output direction is
independent from the TX or RX bit clocks directions (TCR2[BCD] and
RCR2[BCD]). Introduces mclk-output property.

Signed-off-by: Derek Snell <derek.snell@nxp.com>

tests/drivers/i2s/i2s_speed/boards/frdm_mcxn947_mcxn947_cpu0.overlay

@@ -9,3 +9,7 @@
 		i2s-node1 = &sai0;
 	};
 };
+
+&sai0 {
+	mclk-output;
+};