Fix STM32H7Ax build

[multiplemonomials]

Summary of changes

Fix for build errors on STM32H7Ax:

• Typo in targets JSON so clock settings weren't being applied
• CMake looking for nonexistent system_clock.c

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)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

Tested locally, it compiles now!