rt2x00: add support for MT7620

Basic support for MT7620 built-in wireless radio was added to
OpenWrt in r41441. It has seen some heavy cleaning and refactoring
since in order to match the Kernel's code quality standards.

Signed-off-by: Roman Yeryomin <roman@advem.lv>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>