unmaintained/xiaomi-wt88047-downstream: new device (Xiaomi Redmi 2)

This is a follow-up for !2081 (merged), which adds a similar unmaintained device port for xiaomi-wt88047 using the downstream kernel. Most of the debugging-related patches are re-used, some need to be special. I used this to validate the battery/charging driver for mainline (one of the patches adds functionality to dump out all registers of the charger chip for comparison).