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>