soc: arm: add ARM MPU node info and fixup for Cortex-M7 SoCs

Unlike Cortex-M3 and Cortex-M4, in Cortex-M7 the number of
MPU regions may vary based on the implementation. This commit
adds a DTS node for the ARM MPU peripheral in the device tree
of Cortex-M7 SoCs and updates the fixup files, so we may extract
the number of MPU regions at build time. SoCs:
- nxp_rt
- same70
- stm32f7

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>

dts/arm/nxp/nxp_rt.dtsi

@@ -18,6 +18,14 @@
 			device_type = "cpu";
 			compatible = "arm,cortex-m7";
 			reg = <0>;
+			#address-cells = <1>;
+			#size-cells = <1>;
+
+			mpu: mpu@e000ed90 {
+				compatible = "arm,armv7m-mpu";
+				reg = <0xe000ed90 0x40>;
+				arm,num-mpu-regions = <16>;
+			};
 		};
 	};