x86: coreboot: Use a memory-mapped UART

This is much more common on modern hardware, so default to using it.

This does not affect the normal UART, but does allow the debug UART to
work, since it uses serial_out_shift(), etc.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>