npcx/uart: Pull up communication line during transaction
npcx/uart: Pull up communication line during transaction

Boards needs to define GPIO_EC_COMM_PU, which needs to be driven
high during EC-EC transaction. This makes sure that the line does
not go low, even when the base goes away during the transaction.

BRANCH=none
BUG=b:68954760
TEST=EC-EC communication works with this change.
TEST=With signal analyzer, check that UART line is always kept
     high while transaction is in progress.

Change-Id: Iad7b26a9a93b674aa4fff0bc3a72a13e6782515d
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/845544
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec