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

os/src/timer.rs

@@ -0,0 +1,13 @@
+use riscv::register::time;
+use crate::sbi::set_timer;
+use crate::config::CPU_FREQ;
+
+const TICKS_PER_SEC: usize = 100;
+
+pub fn get_time() -> usize {
+    time::read()
+}
+
+pub fn set_next_trigger() {
+    set_timer(get_time() + CPU_FREQ / TICKS_PER_SEC);
+}