ARM: stm32: prepare stm32 family to welcome armv7 architecture

This patch prepares the STM32 machine for the integration of Cortex-A
based microprocessor (MPU), on top of the existing Cortex-M
microcontroller family (MCU). Since both MCUs and MPUs are sharing
common hardware blocks we can keep using ARCH_STM32 flag for most of
them. If a hardware block is specific to one family we can use either
ARM_SINGLE_ARMV7M or ARCH_MULTI_V7 flag.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>