Fix documentation references to moved utility functions

Update platform-api.rst to reference chipflow_lib.utils instead of
deleted chipflow_lib.platform.utils module.

Fixes CI documentation build failure.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: robtaylor

docs/platform-api.rst

@@ -156,9 +156,9 @@ Utility Functions
 
 .. autofunction:: chipflow_lib.platform.base.setup_amaranth_tools
 
-.. autofunction:: chipflow_lib.platform.utils.top_components
+.. autofunction:: chipflow_lib.utils.top_components
 
-.. autofunction:: chipflow_lib.platform.utils.get_software_builds
+.. autofunction:: chipflow_lib.utils.get_software_builds
 
 Constants
 ---------