debug_uart: Add an option to announce the debug UART

It is useful to see a message from the debug UART early during boot so that
you know things are working. Add an option to enable this. The message will
be displayed as soon as debug_uart_init() is called.

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