arch: xtensa: Call the waiti instruction in up_idle()

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

arch/xtensa/src/common/xtensa_idle.c

@@ -92,4 +92,11 @@ void up_idle(void)
   leave_critical_section(flags);
 #endif
 #endif
+
+  /* This would be an appropriate place to put some MCU-specific logic to
+   * sleep in a reduced power mode until an interrupt occurs to save power
+   */
+
+  asm("waiti 0");
+
 }