[xiaomi-elish] Get pen input working

After seeing that the Pad 5 non-Pro (nabu) has pen working in this MR (https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/5804), I cherry-picked the commits required to get the pen working into sm8250 and tested it with my Xiaomi Smart Pen V2. It works! Please merge these for the next release as I'd really like to be able to use the pen.

Thanks to @map220v and @guerrera.nicola for the patches!

The pen charger still is not working, so the pen won't charge when it is magnetically attached to the tablet. On the Nabu MR we discussed this and the IDT P9418 charger IC driver still needs to be ported over. Nicola is working on that and I'd be happy to test on elish when something is available.