stm32l1: Use DBGMCU_IDCODE for part detection

[anonymous0719]

Otherwise STM32L152RC on stm32l152c-discovery is recognized with only 4k

Detailed description

This PR fixes incorrect device identification on the stm32l152c-discovery board.

Currently, STM32L152RC may be detected with an incorrect part_id, which leads to it being recognized with only 4k, causing the wrong memory layout/flash sizing to be used.

The fix is to populate target->part_id by reading the MCU's DBGMCU_IDCODE register (masked to the device ID field), instead of relying on ap->partno. This provides a more reliable and accurate identification for STM32L1 devices. I don’t have the stm32l152c-discovery board available to test locally, but the change is minimal and matches the downstream fix.

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Also as some context: we’ve been developing a tool that scans downstream forks to find valuable commits that haven’t been merged back upstream, and your repo was one of the best examples we found. This commit stood out as a small but important fix, so we manually reviewed it and are submitting it here as a PR in case it’s useful for the upstream project.

If you feel this change is not desired or there’s a better upstream approach, we’d really appreciate any suggestions — both for this patch and for improving our tool’s filtering logic. Thanks for your review!

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