SAME5x direct CDC support

[crabel99]

Summary

• Extend SAME5x support with direct CDC transfer handling.
• Use SAME5x device headers and interrupt definitions through the supported include paths.
• Preserve deferred USB service and DMA completion behavior used by the ArduinoCore-samd integration.

This replaces #585. The original same5x-support branch diverged before the direct-CDC work. This replacement allows CI and review to run against the branch used by the current ArduinoCore-samd integration without rewriting the old public branch.

Validation

Automated validation

• Adafruit TinyUSB Arduino CI: all required architecture and pre-commit checks pass.
• ArduinoCore-samd Metro M0 and Metro M4 example matrices build with this dependency revision.
• The ArduinoCore-samd Metro M4 TinyUSB example sweep passes.

Hardware validation

• Target MCU: SAME54P20A.
• Verified USB reset handling, endpoint-zero setup, address assignment, and host-side CDC enumeration.
• Exercised direct CDC OUT and IN transfers with framed request/response traffic.
• Verified deferred transfer completion and disconnect/reconnect behavior.

Dependency context

• The corresponding ArduinoCore-samd integration is tracked in SERCOM async/DMA with SAME5x support ArduinoCore-samd#395.