Nordic: Support 31250 baud rate

[gutbag]

Summary of changes

Add support for MIDI baud rate 31250 on Nordic nRF52 targets. Fixes #13577

Impact of changes

Migration actions required

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

Tested by setting BufferSerial baud rates to 28800, 31250 & 38840 and checking that receiver sees all characters correctly at each setting.

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Reviewers

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[ciarmcom]

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

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Known issue, we are working on fixing it (internal CI issue).

cc @ARMmbed/mbed-os-test

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