CMake: STM32: Add missing emac init files

[boraozgen]

Summary of changes

Fixes #14568

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

[X] 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

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Reviewers

@jeromecoutant

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[Patater]

Looks complete. Thanks!

$ find . -name stm32f4_eth_init.c
./connectivity/drivers/emac/TARGET_STM/TARGET_STM32F4/TARGET_ARCH_MAX/stm32f4_eth_init.c
./connectivity/drivers/emac/TARGET_STM/TARGET_STM32F4/TARGET_NUCLEO_F429ZI/stm32f4_eth_init.c
./connectivity/drivers/emac/TARGET_STM/TARGET_STM32F4/TARGET_NUCLEO_F439ZI/stm32f4_eth_init.c

[0xc0170]

CI started

[mbed-ci]

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