wifi: rt2x00: add support for external PA on MT7620

Implement support for external PA connected to MT7620A.

Signed-off-by: Tomislav Požega <pozega.tomislav@gmail.com>
[pozega.tomislav@gmail.com: use chanreg and dccal helpers.]
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/af2c68ff831816a86fc39b0c10911c129a1f03dc.1663445157.git.daniel@makrotopia.org