x86: baytrail: Introduce a Kconfig option for the internal UART

There are quite a number of BayTrail boards that uses an external
SuperIO chipset to provide the legacy UART. For such cases, it's
better to have a Kconfig option to enable the internal UART.

So far BayleyBay and MinnowMax boards are using internal UART as
the U-Boot console, enable this on these two boards.

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