mach-stm32: Use default memory map as background region

On linux kernel side, on STM32F7 and STM32H7 SoCs, DMA requires
uncachable regions. These regions are defined in DT.
Since kernel linux v4.15, on ARMv7-M Cortex, kernel is able
to configure MPU regions depending on DT settings.

As kernel is able to configure MPU, this allows to remove
MPU region settings in bootloader.

On Cortex M processors, MPU allows to use a default memory map.
(see B3.5.4 MPU Control Register, MPU_CTRL in
https://developer.arm.com/products/architecture/m-profile/docs/ddi0403/latest/armv7-m-architecture-reference-manual

)
Use the default memory map as background region for all STM32 SoCs
family with an additional MPU region corresponding to the SDRAM area.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>