ec_sync: Add recovery reason for aux update failure

Add a distinct recovery reason for aux firmware update failures
so we have some visibility into failures in eventlog.

This is used in a subsequent commit.

BUG=b:74336712
BRANCH=eve
TEST=manual: force update failure and ensure device goes to recovery
with this reason

Change-Id: I4b215444592b7c31cd25d59ad2a52b85d504e3bf
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://chromium-review.googlesource.com/959669
Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: caveh jalali <caveh@chromium.org>