x86: coreboot: Enable CONFIG_SYS_NS16550_MEM32

The debug UART on modern machines uses a 32-bit wide transfer. Without
this, setting debug output causes a hang or no output. It is not obvious
(when enabling CONFIG_DEBUG_UART) that this is needed.

Enable 32-bit access to avoid this trap.

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