soc: arm: add ARM MPU node info and fixup for ARMv8-M SoCs

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>

dts/arm/nordic/nrf9160.dtsi

@@ -17,6 +17,14 @@
 			device_type = "cpu";
 			compatible = "arm,cortex-m33f";
 			reg = <0>;
+			#address-cells = <1>;
+			#size-cells = <1>;
+
+			mpu: mpu@e000ed90 {
+				compatible = "arm,armv8m-mpu";
+				reg = <0xe000ed90 0x40>;
+				arm,num-mpu-regions = <16>;
+			};
 		};
 	};