soc: stm32: h7: Generate MPU regions from DT nodes

Commit b91d21d3 added the possibility to define MPU regions from the
device tree, however commit c2760885 removed that possibility for H7
SoC, as it now uses a SoC specific definition of the MPU regions without
the DT-defined regions (probably because the two PRs got developped in
parallel).

Fix that by adding the macro which adds the DT-defined regions to the
STM32H7 specific file.

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