rtos: io_drivers: add loopback mode description

This commit adds a description of using SSP devices in loopback mode

Signed-off-by: Marcin Szkudlinski <marcin.szkudlinski@intel.com>

architectures/firmware/sof-zephyr/rtos_layer/io_drivers/i2s/i2s_driver.rst

@@ -108,3 +108,16 @@ stream with already running ones.
 
 "Single" I2S links may be synchronized and aggregated by sending I2sSyncData to
 the I2S IO Driver.
+
+Loopback mode
+======================================
+
+I2S transmitter and receiver share data pins at IP level - the Tx pin of the transmitter 
+is connected to the Rx pin of the receiver. This may be used for easy creation of digital 
+loopbacks (LBM, loopback mode) - the receiver always does see what the sender is sending. 
+
+Please note that 
+ - all the parameters of transmitter and receiver, like number of channels, data rates 
+and format, must match each other
+ - the lines are connected internally, so LBM mode may be used even if the I2S pins are not 
+  physically available