bsp: add probe-rs flash target to CMake for AT32

Add family_flash_probe_rs() and a <target>-probe-rs target for the same
seven AT32 families as the make rule. The jlink target is kept.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011k3HVGfwcxzzqQzqGDrH47
8 files changed