mb/google/octopus: Enable logging of EC wake sources in S0ix

mb/google/octopus: Enable logging of EC wake sources in S0ix

This change adds GSMI callback elog_gsmi_cb_mainboard_log_wake_source
to enable logging of EC wake events in S0ix.

BUG=b:79449585
TEST=Verified that S0ix entry/exit events are added to eventlog:

=========== Lid open ================
62 | 2018-06-25 14:02:36 | S0ix Enter
63 | 2018-06-25 14:02:56 | S0ix Exit
64 | 2018-06-25 14:03:26 | Wake Source | GPE # | 15
65 | 2018-06-25 14:03:32 | Wake Source | GPE # | 65
66 | 2018-06-25 14:03:37 | EC Event | Lid Open

Change-Id: Icc8cd3624966ff66d2cf189871e452cf650cec40
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/27235
Reviewed-by: Justin TerAvest <teravest@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>