kukui_scp: Increase UART TX buffer size to 8192
kukui_scp: Increase UART TX buffer size to 8192

BRANCH=none
BUG=none
TEST=make BOARD=kukui_scp -j

Change-Id: If7defac8ef7e90c0d710e0be8c524bee26813736
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1478534
Reviewed-by: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>

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