Fix USB on DISCO_F769NI

[Jookia]

Summary of changes

Fixes USB device support on the DISCO_F769NI target.
Related to #14564 .

Impact of changes

Migration actions required

Documentation

The existing target code and configuration contains two issues:

• The wrong pins are set to ULPI mode
• The targets.json file does not use ULPI mode

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

I'm not sure what kind of test this needs.
Running the USBCDC example from https://os.mbed.com/docs/mbed-os/v6.15/apis/usbcdc.html works.
Without this patch the device does not even enumerate to Linux.

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Reviewers

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[ciarmcom]

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

[jeromecoutant]

Do you think you can add a commit with the patch for DISCO_H747I ? :-)
https://github.com/ARMmbed/mbed-os/blob/master/targets/TARGET_STM/TARGET_STM32H7/TARGET_STM32H747xI/TARGET_DISCO_H747I/PeripheralPins.c#L550-L551

[Jookia]

I thought about doing that, but I wouldn't be able to test it. The best I can do is check schematics.

[0xc0170]

CI started

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