[coreboot] Deprecate Cavium ThunderX support

[Patrick Rudolph]

Hi folks,
Currently there are only 2 boards using Cavium's ThunderX (CN81xx) and
both aren't available on the free market.
Cavium haven't upstreamed their Arm Trusted Firmware, so it's a bit of
maintenance work to keep it alive.

If nobody wants to take care of if, it will marked deprecated with he
next release (4.11) and removed before the 4.12 release.

Regards,
Patrick Rudolph

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