arch: arm: stm32f7: Kconfig: select CPU_HAS_ARM_MPU

Now that Cortex-M7 cache issues have been fixed in commits 828ae6b8 and
13972693, it is possible to safely enable the MPU on the STM32F7 SoC
series.

Note that the ITCM area is not mapped into an MPU region. This should
not be an issue for now, as Zephyr does not provide yet a way to
populate and use this area.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>