Chapter3: power2/3/5 & sleep test worked on k210/qemu based on timer.

os/src/sbi.rs

@@ -24,6 +24,10 @@ fn sbi_call(which: usize, arg0: usize, arg1: usize, arg2: usize) -> usize {
     ret
 }
 
+pub fn set_timer(timer: usize) {
+    sbi_call(SBI_SET_TIMER, timer, 0, 0);
+}
+
 pub fn console_putchar(c: usize) {
     sbi_call(SBI_CONSOLE_PUTCHAR, c, 0, 0);
 }