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/arch/arm/mach-stm32/Kconfig b/arch/arm/mach-stm32/Kconfig
index b618b60..f4c93f1 100644
--- a/arch/arm/mach-stm32/Kconfig
+++ b/arch/arm/mach-stm32/Kconfig
@@ -38,7 +38,7 @@
 	select STM32_SDRAM
 	select STM32_RCC
 	select STM32_RESET
-	select STM32X7_SERIAL
+	select STM32_SERIAL
 	select SYSCON
 
 source "arch/arm/mach-stm32/stm32f4/Kconfig"