[][mac80211][wifi6][mt76][Add variant support for Cheetah MT76]

[Description]
Add variant support for Cheetah MT76.

[Release-log]
N/A

Change-Id: I4ab0cd04f0c8cb154e67674f205b976cbff9bea0
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8310499
2 files changed