chore: update rust-toolchain to nightly-2025-12-12 (#328)

* Update dependencies and refactor code for improved functionality

- Updated the axstd, axalloc, axconfig, axdisplay, axdriver, axfs, axhal, axipi, axlog, axmm, axnet, and axtask dependencies in Cargo.toml to the latest tag "dev-251216".
- Refactored the boot argument retrieval in the FDT parser to use the updated path for get_bootarg().
- Removed the unused doc_auto_cfg feature in axruntime.
- Changed the Rust toolchain channel from nightly-2025-05-20 to nightly-2025-12-12 in rust-toolchain.toml.
- Updated the zerocopy dependency in xtask/Cargo.toml to remove the specific version constraint and adapt to the new nightly channel.

* chore: update Rust toolchain channel to nightly-2025-12-12

Author: ZR233