nrf52: reset UARTE peripheral in serial_free

[0xc0170]

Description (required)

Resolves: #11758

Taken from #11779, thanks @RobVlaar

Summary of change (What the change is for and why)

Add workaround to reset the UARTE peripheral on a destruction of SerialBase to be able to go into deep sleep.

Documentation (Details of any document updates required)

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Pull request type (required)

[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 (required)

[] 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 (optional)

@desmond-blue @RobVlaar

Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

@0xc0170, thank you for your changes.
@desmond-blue @ARMmbed/mbed-os-maintainers please review.

[kjbracey-arm]

This doesn't match the description.

serial_free isn't being called from the destructor, so I guess this allows reduced power only via enable_input/output?

I still think serial_free should be added to the destructor, but can be a separate task.

[RobVlaar]

@0xc0170 can you change this in SerialBase.cpp on line 276
@kjbracey-arm like this ?

SerialBase::~SerialBase()
{
    // No lock needed in destructor

    // Detaching interrupts releases the sleep lock if it was locked
    for (int irq = 0; irq < IrqCnt; irq++) {
        attach(NULL, (IrqType)irq);
    }
    
    if(_rx_enabled || _tx_enabled))
    {  
        serial_free(&_serial);
    }
}

[0xc0170]

I still think serial_free should be added to the destructor, but can be a separate task.

Yes, we should do it. I thought it was in PR or already on master so I took only nrf changes here

[0xc0170]

@kjbracey-arm shall I add the suggestion to a new PR?

[0xc0170]

I fixed the title, this is just fixing nrf52 serial_free implementation.

@kjbracey-arm please review

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
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