native,devices,audio: add SBSTAT register

* this can expose the status of the stream buffer to applications

am/src/native/devices/audio.c

@@ -68,3 +68,15 @@ size_t __am_audio_write(uintptr_t reg, void *buf, size_t size) {
   }
   return 0;
 }
+
+size_t __am_audio_read(uintptr_t reg, void *buf, size_t size) {
+  switch (reg) {
+    case _DEVREG_AUDIO_SBSTAT: {
+      _DEV_AUDIO_SBSTAT_t *stat = (_DEV_AUDIO_SBSTAT_t *)buf;
+      stat->count = count;
+      stat->bufsize = SBUF_SIZE;
+      return size;
+    }
+  }
+  return 0;
+}