audio/i2s: add i2s_mclkfrequency method to the I2S lower half

This method allows the codec driver to set a specific mclk
frequency for the I2S lower half. This provides support to change
it on run time according to the supported master clock frequency
of the audio codec, the sample rate and the data width.

include/nuttx/audio/i2s.h

@@ -225,6 +225,29 @@
 #define I2S_SEND(d,b,c,a,t) \
   ((d)->ops->i2s_send ? (d)->ops->i2s_send(d,b,c,a,t) : -ENOTTY)
 
+/****************************************************************************
+ * Name: I2S_MCLKFREQUENCY
+ *
+ * Description:
+ *   Set the master clock frequency. Usually, the MCLK is a multiple of the
+ *   sample rate. Most of the audio codecs require setting specific MCLK
+ *   frequency according to the sample rate. NOTE: this parameter may not
+ *   be implemented on I2S driver. If not implemented, the I2S may set
+ *   internally any value to the master clock (or even does not support it).
+ *
+ * Input Parameters:
+ *   dev        - Device-specific state data
+ *   frequency  - The I2S master clock's frequency
+ *
+ * Returned Value:
+ *   Returns the resulting master clock or a negated errno value on failure.
+ *
+ ****************************************************************************/
+
+#define I2S_MCLKFREQUENCY(d,f) \
+  ((d)->ops->i2s_mclkfrequency ? \
+   (d)->ops->i2s_mclkfrequency(d,f) : -ENOTTY)
+
 /****************************************************************************
  * Name: I2S_IOCTL
  *
@@ -286,6 +309,11 @@ struct i2s_ops_s
                             FAR void *arg,
                             uint32_t timeout);
 
+  /* Master Clock methods */
+
+  CODE uint32_t (*i2s_mclkfrequency)(FAR struct i2s_dev_s *dev,
+                                     uint32_t frequency);
+
   /* Ioctl */
 
   CODE int      (*i2s_ioctl)(FAR struct i2s_dev_s *dev,