Modify Gaia power message to match x86 power module

Message from power modules should be similar for easier testing.

BUG=chrome-os-partner:11815
TEST=none

Change-Id: I07e061a34e7d8f8c1f1547d6e15f22f29ee22b84
Reviewed-on: https://gerrit.chromium.org/gerrit/28394
Commit-Ready: Vic Yang <victoryang@chromium.org>
Reviewed-by: Vic Yang <victoryang@chromium.org>
Tested-by: Vic Yang <victoryang@chromium.org>