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/Makefile b/drivers/serial/Makefile
index 7adcee3..5ef603a 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -44,7 +44,7 @@
 obj-$(CONFIG_STM32_SERIAL) += serial_stm32.o
 obj-$(CONFIG_STI_ASC_SERIAL) += serial_sti_asc.o
 obj-$(CONFIG_PIC32_SERIAL) += serial_pic32.o
-obj-$(CONFIG_STM32X7_SERIAL) += serial_stm32x7.o
+obj-$(CONFIG_STM32_SERIAL) += serial_stm32.o
 obj-$(CONFIG_BCM283X_MU_SERIAL) += serial_bcm283x_mu.o
 obj-$(CONFIG_MSM_SERIAL) += serial_msm.o
 obj-$(CONFIG_MVEBU_A3700_UART) += serial_mvebu_a3700.o