STM32G4: corrections for STM32G491xE, STM32G4A1xE and STM32G471xE

[jeromecoutant]

Summary of changes

• MCU_STM32G491xE and MCU_STM32G4A1xE were created in targets.json file:
  https://github.com/ARMmbed/mbed-os/blob/master/targets/targets.json#L3095
  but files were missing in https://github.com/ARMmbed/mbed-os/tree/master/targets/TARGET_STM/TARGET_STM32G4

• TIM instances for STM32G471xx have been changed since latest version of
  https://github.com/STMicroelectronics/STM32_open_pin_data
  This needs some update in the CMSIS file

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

[] No Tests required for this change (E.g docs only update)
[x] 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]

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

[0xc0170]

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[mbed-ci]

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