Increase MSI clock frequency when exiting deep sleep

[LMESTM]

Summary of changes

This will optimize down the time it takes to restore the clock
settings when getting out of deep sleep.

If 48MHz is available let's use it, otherwise at least 4MHz should be
available for any MCU with MSI.

Fixes #12116

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

ST non regression test suite RUN and PASS.

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Reviewers

@jeromecoutant

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[ciarmcom]

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

[0xc0170]

What are the implications, besides getting the time down? power consumption?

[0xc0170]

I started CI meanwhile

[LMESTM]

What are the implications, besides getting the time down? power consumption?

This is a temporary state only - this will save time in wake-up and in the end allow spending more time in low power.

[0Grit]

+1 for this. Normally this sort of effort falls to me and our team to optimize.

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts