[M252KG] Override mpu-rom-end to 0x1fffffff

Without this override, mpu hal will require 5 mpu regions which exceed 4 mpu
regions supported by M252 (see hal/mpu/mbed_mpu_v8m.c). In this scenario,
we will hit assert error but we actually meet stack overrun first due to just
0x400 bytes for emitting error message. The issue doesn’t occur on other
targets such as M487 because it has 8 mpu regions.

Author: ccli8

targets/targets.json

@@ -8325,6 +8325,7 @@
             }
         },
         "overrides": {
+            "mpu-rom-end": "0x1fffffff",
             "tickless-from-us-ticker": true
         },
         "inherits": ["Target"],