serial: stm32x7: Convert CONFIG_STM32X7_SERIAL to Kconfig

Add CONFIG_STM32X7_SERIAL as a Kconfig option.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Vikas Manocha <vikas.manocha@st.com>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 0748a92..2582c95 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -509,6 +509,13 @@
 	  on STiH410 SoC. This is a basic implementation,  it supports
 	  following baudrate 9600, 19200, 38400, 57600 and 115200.
 
+config STM32X7_SERIAL
+	bool "STMicroelectronics STM32 SoCs on-chip UART"
+	depends on DM_SERIAL && STM32F7
+	help
+	  If you have a machine based on a STM32 F7 you can enable its
+	  onboard serial ports, say Y to this option. If unsure, say N.
+
 config MPC8XX_CONS
 	bool "Console driver for MPC8XX"
 	depends on 8xx