Fix interrupt initialization for NRF52 series

[marcuschangarm]

Description

In some cases the UARTE interrupt would be enabled with pending
interrupts. This commit ensures that interrupts are only enabled
from a known state.

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[dlfryar-zz]

+1 validated on the full stack with the NRF52840_DK

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CI job aborted due to CI restart, job restarting now

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Build : FAILURE

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Build : SUCCESS

Build number : 2519
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7401/

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Exporter Build : FAILURE

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Test : SUCCESS

Build number : 2281
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7401/2281

[cmonr]

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Exporter Build : SUCCESS

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