ITE: soc: add cpu idle task

Implement the CPU idle task. The system should enter this task when
there is no any task to ensure power saving.

Tested on it8xxx2_evb board. It will reduce 12.5mA when system enters
the CPU idle task.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>
4 files changed