x86: Allow a UART to be set up before the FSP is ready

Since the FSP is a black box it helps to have some sort of debugging
available to check its inputs. If the debug UART is in use, set it up
after CAR is available.

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