Minor fixes for peripheral pins on Dragonfly Nano

[felser]

Summary of changes

For target device MTS_DRAGONFLY_L471QG:
PG8 should be labeled I2C3 not I2C1.
PC0 is dedicated to measuring system voltage. So remove from UART and I2C.

Impact of changes

Migration actions required

Documentation

None.

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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]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts