Explicitly set tdb internal flash size for PSOC6

The default computation assumes that a flash sector is several times
larger than a flash page. On PSoC 6 targets this is not the case
(the two values are the same) so the computed size is too small.
This is a similar change to 1b1f14d36b,
but for devices which implement TDB in internal storage.

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

@@ -22,6 +22,9 @@
         "FVP_MPS2": {
             "internal_size": "0x200000",
             "internal_base_address": "0x00200000"
+        },
+        "MCU_PSOC6": {
+            "internal_size": "7168"
         }
     }
 }