set MPIE and MPP

Signed-off-by: liangkangnan <liangkangnan@163.com>

tests/example/init.c

@@ -16,5 +16,9 @@ void trap_handler(uint32_t mcause)
 
 void _init()
 {
+    // 设置中断入口函数
     write_csr(mtvec, &trap_entry);
+    // 使能CPU全局中断
+    // MIE = 1, MPIE = 1, MPP = 11
+    write_csr(mstatus, 0x1888);
 }