serial: atmel_usart: Support enable an early debug UART

Add support to enable an early debug UART for debugging.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 9e48902..507a27d 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -98,6 +98,13 @@
 	  driver will be available until the real driver model serial is
 	  running.
 
+config DEBUG_UART_ATMEL
+	bool "Atmel USART"
+	help
+	  Select this to enable a debug UART using the atmel usart driver. You
+	  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_NS16550
 	bool "ns16550"
 	help