Expose SerialBase IrqType member to UnbufferedSerial class

[rajkan01]

Summary of changes

UnbufferedSerial class is privately inherited from Serialbase class, so SerialBase RxIrq, TxIrq type members are exposed by using C++ directive which is required argument for attach(Callback func, IrqType type = RxIrq) API.

Impact of changes

With these changes, SerialBase class RxIrq, TxIrq members can be accessible.

Migration actions required

None.

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

@evedon

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[ciarmcom]

@rajkan01, thank you for your changes.
@evedon @ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

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CI started

[mbed-ci]

Test run: SUCCESS

Summary: 7 of 7 test jobs passed
Build number : 1
Build artifacts