Implemented recommended fix from https://github.com/STMicroelectronic…

[J91Olivier]

The baud rate was calculated incorrectly for STM32F4 targets that have UART9 and UART10 peripherals. A fix has been implemented as recommended from STMicroelectronics/STM32CubeF4#5.

Summary of changes

Ensures that the baud rate register value is calculated correctly for STM32F4 targets that have UART9 and UART10 peripherals.

When the targets clock frequency is configured such that APB1 is different from APB2 the baud rates for UART9 and UART10 will not be calculated correctly and the device will not have its expected baud rate when interfacing with other devices. This fixes #12090

Impact of changes

There are no implications for users that were not affected by this bug. It will only execute the fix for the devices that have a UART9 and UART10 peripheral.

Migration actions required

No mitigation actions are required for this fix.

Documentation

None

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

@jeromecoutant

[ciarmcom]

@J91Olivier, thank you for your changes.
@jeromecoutant @ARMmbed/mbed-os-maintainers please review.

[jeromecoutant]

ST_INTERNAL_REF 50651

[kjbracey]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 12 of 12 test jobs passed
Build number : 1
Build artifacts