kevin: Increase UART TX buffer size to 4kb
kevin: Increase UART TX buffer size to 4kb

We have enough memory for that, and it makes it possible to poll the
logs from AP much more unfrequently.

BRANCH=none
BUG=chrome-os-partner:56460
TEST=make buildall -j
TEST=Boot kevin, cat /sys/kernel/debug/cros_ec/console_log does not
     miss any data.

Change-Id: I7dc880a27d34f97746a8fde00e49d8d08ed85b9d
Reviewed-on: https://chromium-review.googlesource.com/373285
Commit-Ready: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Tested-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Gwendal Grignou <190c5a5edfcf62e0246559bf31fa7548674de3ad@chromium.org>
Reviewed-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>

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