lwIP | Add GRETH/GRETH_GBIT driver (!40)

[Matteo Concas (@matteo.concas)]

Matteo Concas created a merge request: https://gitlab.rtems.org/rtems/pkg/rtems-lwip/-/merge_requests/40

Project:Branches: matteo.concas/rtems-lwip-greth:matteo/greth to rtems/pkg/rtems-lwip:main
Author:   Matteo Concas



## Summary

See !22 for full history of the original work done during GSoC 2025 by Prithvi Tambewagh.

Building up on his work I have made multiple improvements:
- Format source code so it matches RTEMS kernel format
- Bug fixes
- Optimize driver configuration for throughput
- Take into account checksum offloading
- Only trigger TX interrupts when necessary (75%+ of descriptors used)
- Replace hardcoded bitfields with defines to improve readibility
- Make it possible to pick GRETH device instance

The driver has been tested on hardware using TTCP and iperf3 (or a crude port of it at least).

One improvement I can see is figuring out a way to enable SW checksum generation only when an outgoing packet exceeds the MTU (currently it is always enabled). The GRETH core supports checksum offloading only for non-fragmented packets.
Figuring this out would provide a pretty good increase in throughput.

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