Fix messed up CMake targets for NUCLEO_H745ZI_Q

[multiplemonomials]

Summary of changes

The CMake build files for NUCLEO_H745ZI_Q were quite messed up -- it never added the subdirectory, and some of the target names had typos so it couldn't actually build properly. This fixes the buildfile issues and condenses down the CMake targets to remove duplicated code.

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)
[X] Tests / results supplied as part of this PR

Built locally for NUCLEO_H745ZI_Q_CM7 and it builds OK!

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