[libcpu][update]重启与关机函数：rt_hw_cpu_shutdown、rt_hw_cpu_reset，补充WEAK属性

libcpu/mips/gs232/cpuport.c

@@ -23,7 +23,7 @@
  * this function will reset CPU
  *
  */
-void rt_hw_cpu_reset(void)
+RT_WEAK void rt_hw_cpu_reset(void)
 {
     /* open the watch-dog */
     WDT_EN = 0x01; 		/* watch dog enable */
@@ -38,7 +38,7 @@ void rt_hw_cpu_reset(void)
  * this function will shutdown CPU
  *
  */
-void rt_hw_cpu_shutdown(void)
+RT_WEAK void rt_hw_cpu_shutdown(void)
 {
     rt_kprintf("shutdown...\n");