STM32: update SetSysClock for NUCLEO_L476RG

[jeromecoutant]

Summary of changes

Default configuration for clock source for NUCLEO_L476RG is MSI.

Issue came from forum:
https://forums.mbed.com/t/thisthread-sleep-for-not-returning-in-mbed-os-6-with-hse-xtal/10352

HSE configuration was not correct.
After waking up from deep sleep, clock configuration was blocked by MSI.

Impact of changes

None for default HW configuration

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/mbed-os-maintainers please review.

[adbridge]

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[mbed-ci]

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