risc-v: Add WFI to up_idle and fix compile issue

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

arch/risc-v/src/common/riscv_idle.c

@@ -53,4 +53,13 @@ void up_idle(void)
    */
 
   nxsched_process_timer();
+#else
+
+  /* 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("WFI");
+
+#endif
 }