serial: stm32: Rename serial_stm32x7.c to serial_stm32.c

Now this driver is used across stm32f4, stm32f7 and stm32h7
SoCs family, give it a generic name.

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 122b8e7..7b20b47 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -529,7 +529,7 @@
 	  on STiH410 SoC. This is a basic implementation,  it supports
 	  following baudrate 9600, 19200, 38400, 57600 and 115200.
 
-config STM32X7_SERIAL
+config STM32_SERIAL
 	bool "STMicroelectronics STM32 SoCs on-chip UART"
 	depends on DM_SERIAL && (STM32F4 || STM32F7 || STM32H7)
 	help