Remove ITM from NRF52_DK and DELTA_DFBM_NQ620 targets

[TacoGrandeTX]

The NRF52_DK and DELTA_DFBM_NQ620 have the SWO pin (p0_18) mapped to LED2. This means that on startup LED2 turns on after the ITM is initialized which is confusing. Since most users want LED2 usage
instead of SWO we remove the ITM for these targets.

The nRF52 readme is updated to instruct users how to get SWO support if they need it.

This addresses #9695

Description

By default LED2 is now off on startup and ITM/SWO functionality can be regained by bringing in ITM through mbed_app.json as documented in updated nRF52 readme:

    "target_overrides": {
        "*": {
            "target.device_has_add": ["ITM"]
        }
    }

Tested that LED2 is now off on entry to main() and SWO capability can be brought back in through mbed_app.json.

Pull request type

[ ] Fix
[ ] Refactor
[x] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

@dlfryar @pan-

[ciarmcom]

@TacoGrandeTX, thank you for your changes.
@dlfryar @pan- @ARMmbed/mbed-os-maintainers please review.

[pan-]

Do you know if NRF52840_DK is impacted ?

[0xc0170]

CI started

[TacoGrandeTX]

@pan- On the nRF52840 SWO is mapped to pin P1.00 and it can also be a GPIO. Luckily on the DK board this pin isn't tied to anything and it just goes to header P24.

[mbed-ci]

Test run: SUCCESS

Summary: 12 of 12 test jobs passed
Build number : 1
Build artifacts

[dlfryar-zz]

+1 looks good

[cmonr]

#9767 (comment)

@pan- All good?

[pan-]

@TacoGrandeTX If I look at the pin mapping of the nRF52840_DK; it appears that BUTTON3 is also mapped on P0.24. Is there any side effect we should consider ?

[TacoGrandeTX]

@pan- We don't have any contention for the buttons and LEDs on the nRF52840_DK, so we're good with those pins. If we were concerned about external ETM trace then they would lose Button 1 and Button 2 on P0.11 and P0.12 but that's all I see in the nRF52840 Product Spec.

[cmonr]

@pan- I think we're good to merge. Let us know if that isn't the case.