Add support for Teensy 4.0

[rjongeneelen]

The latest version of Ethernet works well with a W5500 on my Teensy 3.6, but not at all with the same W5500 on my Teensy 4.0 (hardware not supported error).

Within this pull request, I've added support for Teensy 4.0 by adding__IMXRT1062__ specific implementations for initSS / setSS / resetSS.

After these additions, I was able to run the WebClient example with DHCP on my Teensy 4.0 without any problems.

[PaulStoffregen]

Please also send this pull request to my copy of Ethernet.

https://github.com/PaulStoffregen/Ethernet

I no longer have write access to Arduino's repository, so I'm back to maintaining my own fork.

[rjongeneelen]

Of course @PaulStoffregen, done.

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