RX6800/RX6800XT/RX6900 support

The big navi cards are currently showing very limited configuration possibilities in CoreCtrl.

Tested on:

• AMD RX6800
• CoreCtrl 1.1.1
• Kernel 5.10.0
• Mesa 20.2.3
• linux-firmware 20201118

Available options and ranges on tested system:

• Core clocks can be set to 500MhZ or 2475MhZ (no in-between or values above 2475).
  • AMD Adrenalin allows 2600 as max.
• Memory clocks can be set to 96, 456, 673 or 1000MhZ (no in-between or values above 1000).
  • AMD Adrenalin allows 1150 as max.
• Power limit range is correct at 1 to 233 watts.
• Core voltage adjustment is not possible.