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/configs/stm32f746-disco_defconfig b/configs/stm32f746-disco_defconfig
index 0daadde..439b796 100644
--- a/configs/stm32f746-disco_defconfig
+++ b/configs/stm32f746-disco_defconfig
@@ -43,6 +43,7 @@
 CONFIG_PINCTRL_STM32=y
 CONFIG_RAM=y
 CONFIG_STM32_SDRAM=y
+CONFIG_STM32X7_SERIAL=y
 CONFIG_DM_SPI=y
 CONFIG_STM32_QSPI=y
 CONFIG_OF_LIBFDT_OVERLAY=y