rt2x00: add RT3883 support

Patch add support for RT3883 chip. Code was taken direclty
from openwrt project and merge into one patch.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>