Fix serial_format() on MIMXRT1170 as well

[multiplemonomials]

Summary of changes

This imports the second change from ARMmbed#15382

Impact of changes

This should fix serial_format() on MIMXRT1170. It was obviously broken due to storing a 32-bit register value in an 8-bit variable.

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)
[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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