Enable FlashIAP for MCU_NRF52832

The NRF52832 MCU supports flash which is declared in `targets.json`
but the declaration of FlashIAP was missing. Applications were unable
to access the MCU's internal flash.

For example, with Mbed OS PSA enabled for NRF52_DK, Mbed CLI 1's
configuration check fails:

    [ERROR] library 'psa-services' requires 'flashiap-block-device' which is not present

because Mbed OS PSA requires internal flash.

Author: LDong-Arm

targets/targets.json

@@ -5898,6 +5898,9 @@
             "TRNG",
             "USTICKER"
         ],
+        "components": [
+            "FLASHIAP"
+        ],
         "extra_labels": [
             "NORDIC",
             "NRF5x",