pd: Allow EC to request PD hibernate via host command

On some systems, we may wish to have the PD follow the EC into
hibernate. Add a status field to EC_CMD_PD_EXCHANGE_STATUS to support
this.

BUG=chrome-os-partner:45010
TEST=Manual on glados with subsequent commit. Run 'hibernate' on EC
console, verify that both EC and PD go to hibernate. Plug zinger and
verify that both EC and PD wake, AP boots, and battery begins charging.
BRANCH=None

Change-Id: I0476bc8a47ffb0fe113dccda9d4f8074105c1c84
Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/302712
Commit-Ready: Shawn N <shawnn@chromium.org>
Tested-by: Shawn N <shawnn@chromium.org>
Reviewed-by: Shawn N <shawnn@chromium.org>
9 files changed