msm-modem: Mobile Data and ModemManager for MSM8916/MSM8974!

This is a collection of commits that make the modem largely working on MSM8916 (and partially on MSM8974).

Shortly said, the two main new features are:

1. Mobile Data: A shiny new "BAM DMUX" kernel driver enables MSM8916 and MSM8974 to talk to the network interface(s) in the modem. This allows using mobile data with both oFono and ModemManager.
2. ModemManager can now talk to the modem via rpmsg channels, which allows it to work on MSM8916 and MSM8974. For now ModemManager is forked to add the patch, but I have also submitted it upstream.
   • Like oFono, Voice Calls (with audio), SMS and Mobile Data should work with ModemManager.
   • I expect this to have no effect on the PinePhone, but it would be good if someone can make sure that ModemManager is still working fine on the PinePhone with these changes.

This is done in 4 commits with small changes in most involved components, see the commit log for details.

As far as I know, mobile data has been successfully tested on the following devices:

• bq-paella (only ModemManager, mobile data does not work in oFono for some reason)
• samsung-a5ulte (both ModemManager and oFono)
• samsung-serranovelte (only ModemManager tested)
• xiaomi-wt88047 (only ModemManager tested, with addditional dual SIM hacks)
• wileyfox-crackling (oFono, ModemManager with additional dual SIM hacks, by @TravMurav)
• motorola-harpia (oFono, ModemManager with even more dual SIM hacks, by @srxl and @TravMurav)
• fairphone-fp2 (MSM8974, only oFono tested, by @z3ntu)

But more testing is always appreciated (and probably necessary), modems behave really weird sometimes :)

Cc: @minlexx @ichernev @lazzardo