arm: zynq: Support the debug UART

Add support for the debug UART to assist with early debugging. Enable it
for Zybo as an example.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 53b4e1b..d462244 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -91,6 +91,13 @@
 	  will need to provide parameters to make this work. The driver will
 	  be available until the real driver-model serial is running.
 
+config DEBUG_UART_ZYNQ
+	bool "Xilinx Zynq"
+	help
+	  Select this to enable a debug UART using the serial_s5p driver. You
+	  will need to provide parameters to make this work. The driver will
+	  be available until the real driver-model serial is running.
+
 endchoice
 
 config DEBUG_UART_BASE