STM32G4: enable QSPI for custom boards

[pilotak]

Summary of changes

Brings support of QSPI for custom boards based on STM32G4
Fixes #15235

Impact of changes

Migration actions required

Documentation

based on #14392

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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)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@jeromecoutant

[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

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