Variable i2s bit width (#331)

- Add support for variable width I2S (via XUA_I2S_N_BITS)
- Add support for variable width TDM (again via XUD_I2S_N_BITS when XUA_PCM_FORMAT=XUA_PCM_FORMAT_TDM)
- Includes support for xcore as I2S/TDM master and slave
- Add testing of the the above to test_i2s_loopback
- Rationalised test config building in test_i2s_loopback
- Documentation updated

lib_xua/api/xua_conf_default.h

@@ -93,7 +93,11 @@
 
 #define XUA_PCM_FORMAT_I2S      (0)
 #define XUA_PCM_FORMAT_TDM      (1)
-
+/**
+ * @brief Format of PCM audio interface. Should be set to XUA_PCM_FORMAT_I2S or XUA_PCM_FORMAT_TDM
+ *
+ * Default: XUA_PCM_FORMAT_I2S
+ */
 #ifdef XUA_PCM_FORMAT
     #if (XUA_PCM_FORMAT != XUA_PCM_FORMAT_I2S) && (XUA_PCM_FORMAT != XUA_PCM_FORMAT_TDM)
         #error Bad value for XUA_PCM_FORMAT
@@ -193,6 +197,19 @@
     #define I2S_DOWNSAMPLE_CHANS_IN I2S_CHANS_ADC
 #endif
 
+/**
+ * @brief Number of bits per channel for I2S/TDM. Supported values: 16/32-bit.
+ *
+ * Default: 32 bits
+ */
+#ifndef XUA_I2S_N_BITS
+#define XUA_I2S_N_BITS (32)
+#endif
+
+#if (XUA_I2S_N_BITS != 16) && (XUA_I2S_N_BITS != 32)
+#error Unsupported value for XUA_I2S_N_BITS (only values 16/32 supported)
+#endif
+
 /**
  * @brief Max supported sample frequency for device (Hz). Default: 192000
  */