serial: stm32: Add debug uart support

Add support for early debug printf, before the availability of
driver model and device tree support.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 8777909..2940bd0 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -324,6 +324,15 @@
 	  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_STM32
+	bool "STMicroelectronics STM32"
+	depends on STM32_SERIAL
+	help
+	  Select this to enable a debug UART using the serial_stm32 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_UNIPHIER
 	bool "UniPhier on-chip UART"
 	depends on ARCH_UNIPHIER