Update FPGA documentation

[gpsimenos]

Summary of changes

This PR updates the command to run FPGA tests in documentation and adds info about the FPGA_FORCE_ALL_PORTS macro.

Impact of changes

Migration actions required

Documentation

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Pull request type

[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

[X] 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

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[ciarmcom]

@gpsimenos, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

@gpsimenos Can you squash into one commit all three?

I could not restart the travis, rebasing will force it to restart anyway.

Pull request has been modified.

[gpsimenos]

@gpsimenos Can you squash into one commit all three?

I could not restart the travis, rebasing will force it to restart anyway.

Done

[0xc0170]

Travis green, starting CI

[MarceloSalazar]

@0xc0170 please lets get this merged :)