EFM32: RTCC bugfix for #12374

[stevew817]

RTC unit tests were failing on devices that use RTCC for both RTC and LP Ticker implementation.

Summary of changes

Now using an RTCC retention register to keep track of user timebase for RTC API. RTC and LP Ticker implementations use the same counter, but they shouldn't share timebases.

Closes #12374.

Documentation

None

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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

@amq

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[ciarmcom]

@stevew817, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

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

Failed test jobs:

• jenkins-ci/mbed-os-ci_cloud-client-pytest

[0xc0170]

client test was restarted

[0xc0170]

I had to restarted again client test, was green but failed in jenkins somewhere