Add default TDBStore location for DISCO_H747I (end of BANK1)

Place it at the end of the bank1, last two erase sectors.
As erase sector is 128 kB, default size must be double of that.

This can't be in bank2 as that is apparently dedicated to the M4 core.

Memory map is available in;

https://github.com/ARMmbed/mbed-os/blob/master/targets/TARGET_STM/TARGET_STM32H7/TARGET_STM32H747xI/flash_data.h

Memory map does not have this information, but issue [11617](https://github.com/ARMmbed/mbed-os/issues/11617) has.

features/storage/kvstore/conf/tdb_internal/mbed_lib.json

@@ -15,6 +15,10 @@
             "internal_size": "0x8000",
             "internal_base_address": "0x00028000"
         },
+        "DISCO_H747I": {
+            "internal_size": "256*1024",
+            "internal_base_address": "0x080C0000"
+        },
        "ARM_MUSCA_A1_S": {
             "internal_size": "0x8000",
             "internal_base_address": "0x00420000"