ramips: mt7620: select only the matching mt76 driver

ramips: mt7620: select only the matching mt76 driver

Because every device has selected the corresponding mt76 driver, we can
now disable the mt76 metapackage by default to make sure that other
devices (those don't need mt76) avoid selecting unwanted packages.

We can find the hardware specifies and determine the dependencies on
these sites:
  https://wikidevi.com/wiki/
  https://openwrt.org/toh/hwdata/

Signed-off-by: Chen Minqiang <ptpt52@gmail.com>
(backported from 40692f0fb55ca5bd2426701b3a9e64e3b7487a7d)