mt76: add driver code for MT76x2u based devices

MT76x2u is a 2x2 USB 802.11ac chipset by MediaTek. This driver currently
support station mode

Tested-by:  <cug_yangyuancong@hotmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>