Re-enable flash clock test for NRF52 but with higher tolerance

[marcuschangarm]

Description

The flash clock test is disabled for the NRF52 series. This change
re-enables the test but with a higher tolerance to accommodate the
high jitter on the current ticker implementation.

Pull request type

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

[marcuschangarm]

@cmonr @0xc0170

This should be better than disabling the test completely.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1870
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6756/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1517
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6756/

[mbed-ci]

Test : SUCCESS

Build number : 1681
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6756/1681