nRF52840: Set mpu-rom-end to 0x1fffffff

So we have at least one test platform exercising the special case of
mpu-rom-end being 0x1fffffff, set that for nRF52840.

Author: kjbracey

targets/targets.json

@@ -6678,6 +6678,9 @@
                 "value": 0
             }
         },
+        "overrides": {
+            "mpu-rom-end": "0x1fffffff"
+        },
         "OUTPUT_EXT": "hex",
         "is_disk_virtual": true,
         "supported_toolchains": ["GCC_ARM", "ARM", "IAR"],