soc: arm: stm32f4: group stm32f412xx in a Kconfig for the serie

A new stm32f412vx devices is introduced.
The soc devices stm32f412cx, stm32f412vx, stm32f412zx are
removed to have a more generic stm32f412xx Kconfig.
The stm32cube modules stm32f412cx/vx/zx exists.

Signed-off-by: Francois Ramu <francois.ramu@st.com>