x86: fsp: Wrap setup_internal_uart() call with CONFIG_INTERNAL_UART

For any FSP-enabled boards that want to enable debug UART support,
setup_internal_uart() will be called, but this API is only available
on BayTrail platform. Change to wrap it with CONFIG_INTERNAL_UART.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>