This commit adds a DTS node for the ARM MPU peripheral in the
device tree of ARMv8-M SoCs (for the secure and the non-secure
DTS descriptions) and updates the fixup files. SoCs:
- nrf9160
- musca_a
- musca_b1
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>