Add board defs for sparkfun board variants of RP2040

[Wenn0101]

Summary of changes

Add support for Sparkfun variants of RP2040. Not 100% if I am creating a PR into the correct branch, but I have created targets for the RP2040 boards that are made by SparkFun.

All three targets can be found on this page:
https://www.raspberrypi.org/documentation/rp2040/getting-started/

With more information on these SparkFun product pages:
https://www.sparkfun.com/products/17717
https://www.sparkfun.com/products/17745
https://www.sparkfun.com/products/17720

Please let me know if you are interested in taking these changes and adding these boards to this branch. We are also working on a porting these targets to arduino using the mbed arduino core .

Impact of changes

Add 3 new targets:
SFE_RP2040_PRO_MICRO
SFE_RP2040_THING_PLUS
SFE_RP2040_MICROMOD

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

@facchinm

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[facchinm]

Hi @Wenn0101 , sorry for the late response.
I'll be super happy to merge these but we still need to smooth some rough edges of the porting (and pass all the tests) before making it official, so I'd rather wait a bit and merge when we are ready for prime time.