msm8916: upgrade to Linux 5.15 and introduce more flexible rproc selection

Add a replacement for the current mainline and mainline-modem kernel variants used for the MSM8916 devices. At the moment this is implemented using separate DTBs that are all maintained in the kernel.

Unfortunately, this does not seem likely to be accepted upstream in the mainline kernel. They will expect to have only a single DTB that enables all available functionality. The "no-modem" variant is a minor but still very useful use case because it changes the audio routing: It allows audio to work without any firmware, with more control on sample rates, latency etc for advanced audio use cases. However, having a potentially enabled modem requires routing all audio through the modem. There are two separate DTBs needed for these two setups.

To avoid burdening mainline with this use case, the idea is to apply the necessary DTB transformations as part of the lk2nd boot process. Mainline only contains the DTB with the full functionality (audio routed through modem). The soc-qcom-msm8916-rproc packages place a magic file at /boot/lk2nd_rproc_mode that is read by lk2nd. When the file contains no-modem lk2nd will transform the DTB to disable the modem and change the audio routing to bypass the modem.

That way, this feature does not need to be considered in mainline at all but can still be offered as "nice to have" by lk2nd. It also simplifies the device packages because the selection is now common as part of the soc-qcom-msm8916 package.

Together with a related pmbootstrap change (pmbootstrap!2132 (merged)) there is a nice prompt for selecting one of these "rproc" providers:

 $ pmbootstrap init
 [...]
 Available providers for soc-qcom-msm8916-rproc (3):
  * all: Enable all remote processors (audio goes through modem) (**default**)
  * no-modem: Disable only modem (audio bypasses modem, ~80 MiB more RAM)
  * none: Disable all remote processors (no WiFi/BT/modem, ~90 MiB more RAM)
 Provider [default]: ...