K82F: Fix serial_api serial_init_clock implicit declaration error

[mmoehabb]

Summary of changes

Fixes #15539 by declaring serial_clock_init function at the top of serial_api file.

Impact of changes

mbed compile -t GCC_ARM -m K82F successfully compiles.

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

[x] 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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