mt76: introduce mt76x2-common module

In order to remove usb dependency from pcie code, add mt76x2-common as
a container of shared code between mt76x2 and mt76x2u. Add eeprom code
to mt76x2-common module

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