x86: Support debug UART in 64-bit mode

The debug UART is already set up in SPL, so there is no need to do
anything here. We must provide the (empty) function though.

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

arch/x86/cpu/x86_64/cpu.c

@@ -50,3 +50,10 @@ int x86_cpu_init_f(void)
 {
 	return 0;
 }
+
+#ifdef CONFIG_DEBUG_UART_BOARD_INIT
+void board_debug_uart_init(void)
+{
+	/* this was already done in SPL */
+}
+#endif