On Seaboard the UART and SPI interfere with each other. This causes the UART
to receive spurious zero bytes after SPI transactions and also means that
SPI can corrupt a few output characters when it starts up if they are still
in the UART buffer.

This updates the board to use the SPI/UART switch to avoid the problem.

For now this feature is turned off since it needs changes to the NS16550
UART to operate.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>