board: arm: Add google_kukui board

This adds support for the EC (embedded controller) on a Google
reference board with codename "kukui". This board uses the STM32F098RC
chip. We built an application for the board and verified UART
functionality on the board.

Signed-off-by: Jack Rosenthal <jrosenth@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>