edgetpu: add user_vote for user space request TPU throttling

For the current upstream design, userspace can request
cooling device via "cur_state" sysfs. However, the request
will be overwritten by other kernel governor or it will
overwrite other governor's request. So, create this user_vote
sysfs for userspace to request cooling device and vote with
other kernel governor's request.

Bug: 188701596
Test: build
Change-Id: I6ea5c1a5f5fdeb72f78f69fa7cc833f5f464d28e
Signed-off-by: TeYuan Wang <kamewang@google.com>