Adding support for RT-AC86U

@slh
i think it's the time to consider this device again. it just like a combination of GT-AC5300 and R8000P (openwrt already supported them), that should make it easier to port. and from wikidevi . it is using BCM4366E + BCM4365E, which are both supported by brcmfmac. 2.4g and 5g should just works (not like archer c2300)