STM32H7: set ADC PLL clock for various source configurations

[facchinm]

Replaces c65d254

Summary of changes

Impact of changes

Migration actions required

Documentation

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Pull request type

[] 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

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Reviewers

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[facchinm]

@manchoz can you give it a spin on x8 / h7 ?

[manchoz]

@facchinm LGTM 👍

Tested both with X8 and H7 with Portenta Breakout Carrier.