Fix missing SYSCFG_CLK_ENABLE() for STM32H7 Ethernet

[multiplemonomials]

Summary of changes

I found that, with master branch of Mbed, Ethernet did not work on STM32H723. After some debugging and A/B testing, I narrowed it down to this line, which I somehow lost when porting code over from the STM32Cube HAL.

Oddly, without this line, Ethernet still manages to work on STM32H743 and H745/H747. However, it completely can't initialize on STM32H72x.

I also suspect this might be related to a very intermittent issue I've been seeing on STM32H747 where Ethernet sometimes loses packets. However, I'm not totally sure.

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

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

After re-adding this line, Ethernet is able to initialize on NUCLEO_H723ZG. Without it, it fails with a timeout while resetting the MAC (likely because the RMII clocking isn't set up correctly).

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