Increase clock tolerance in Flash HAL test

[marcuschangarm]

Description

The current 0.1% clock tolerance is too small for certain platforms
which natural variance is larger than this. This commit increases the
tolerance to 0.5% instead.

Sample output from time_cpu_cycles called repeatedly in init,
before running any flash tests:

[1515706585.63][CONN][RXD] diff: 49316
[1515706585.69][CONN][RXD] diff: 49256
[1515706585.75][CONN][RXD] diff: 49286
[1515706585.81][CONN][RXD] diff: 49256
[1515706585.87][CONN][RXD] diff: 49225
[1515706585.94][CONN][RXD] diff: 49286
[1515706585.99][CONN][RXD] diff: 49317
[1515706586.06][CONN][RXD] diff: 49255
[1515706586.12][CONN][RXD] diff: 49286
[1515706586.18][CONN][RXD] diff: 49285
[1515706586.24][CONN][RXD] diff: 49286
[1515706586.31][CONN][RXD] diff: 49347
[1515706586.36][CONN][RXD] diff: 49347
[1515706586.43][CONN][RXD] diff: 49286
[1515706586.49][CONN][RXD] diff: 49286
[1515706586.55][CONN][RXD] diff: 49256
[1515706586.61][CONN][RXD] diff: 49286
[1515706586.68][CONN][RXD] diff: 49346
[1515706586.74][CONN][RXD] diff: 49347
[1515706586.80][CONN][RXD] diff: 49256

Notice the outliers will cause intermittent CI failures.

[marcuschangarm]

@c1728p9

[c1728p9]

Thanks for investigating this @marcuschangarm

[marcuschangarm]

@studavekar we need this PR before adding the NRF52840_DK to the test farm.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 844
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5831/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 512
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/5831/

[mbed-ci]

Test : SUCCESS

Build number : 688
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5831/688

[studavekar]

@studavekar we need this PR before adding the NRF52840_DK to the test farm.

Thanks Marcus for the PR, CI looks green and is ready for merge