Fix array overflow in gpio configuration code for NRF5x

[marcuschangarm]

Description

Reintroduce PR #6021 which was accidentally removed by PR #6711

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2161
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7022/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 1951
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7022/1951

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1787
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7022/

[changmarcus]

This should not go in 5.8.

[cmonr]

We realized during gatekeeping that there will be a subset of PRs coming in now that should target 5.9.1, but realized this after double tagging most PRs.

I'm scrubbing the current set of double-tagged PRs to make sure this exact thing doesn't happen with them as well :)