STM32H7: system power supply configuration

[jeromecoutant]

Summary of changes

In current code, system power supply is configured in SetSysClock function,
which is not convenient for custom board with a different power supply than the NUCLEO or DISCO default board.

Now it is configurable via json.

@facchinm

Impact of changes

Migration actions required

Documentation

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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

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[ciarmcom]

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

[0xc0170]

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[mbed-ci]

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