[DC21X4] Add driver for DECchip 21x4-compatible network adapters (#5614)

These adapters were common in DEC Alpha boxes and they are really rare
nowadays. The 21140 chip is emulated in Connectix / M

[DC21X4] Add driver for DECchip 21x4-compatible network adapters (#5614)

These adapters were common in DEC Alpha boxes and they are really rare
nowadays. The 21140 chip is emulated in Connectix / Microsoft Virtual PC
and Hyper-V Gen 1 VM.

This is an experimental driver, not yet tested on real hardware.

CORE-8724

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