I just finished doing my own porting of the "mir3p" (aka "xiaomi 3 pro" or "r3 pro") to openwrt, and came across this post (and @dissent1's message that shows someone else has already done the work... and back in February!)
@dissent1: I swear I copied/paste'd the code myself without looking at yours but I might borrow from you if yours looks nicer. For starters, in your .dts file I think this is wrong:
190 mediatek,portmap = "llllw";
that would imply that there are 4 lan ports and one wan, whereas there are actually only 3 lan ports on the R3P (Pandorabox botched this completely, blindly copying the config for the R3G which has the opposite LAN/WAN ordering and only 2 lan ports, with the result that the wan port is garbage and lan port 3 functions as "wan").
As for wifi driver support, @Nossiac has frequent binary-only module builds for the mt7615 (https://github.com/Nossiac/mtk-openwrt-feeds/). I can confirm that the module loads (and wlan interfaces are initialized), and there do seem to be some open bugs, but I'm too lazy to configure/investigate that at the moment. For those who are having issues with Pandorabox (I was) and would rather not use the stock Xiaomi firmware, I think at least a base openwrt device support (with instructions for how to use @Nossiac's binary modules) would be useful (at least, I would use it if someone else had already done it).
Now if I can only figure out who to see about getting code reviewed....