rambi: Fix driving keyboard column 2

Rambi's Silego chip inverts the pull as well as the signal.  So it has
a pulldown for the signal instead of a pullup.  The EC must drive the
signal push-pull, since open-drain only works for signals with pullups.

BUG=chrome-os-partner:23198
BRANCH=none
TEST=type on keyboard; C/D/E/3/F1-F4 should all work

Change-Id: Iaea4f540c523824571a568b70cca3e0fb467f79c
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/172915
Reviewed-by: Aaron Durbin <adurbin@chromium.org>