msm89x7: Add mainline kernel and devices

This MR contains initial mainline support for msm89x7 (msm8917, msm8937, msm8940) devices.

New Packages:

• linux-postmarketos-qcom-msm89x7
• soc-qcom-msm89x7

New Devices:

• qcom-msm89x7

Ported from downstream:

• xiaomi-santoni (replaced with generic package)
• xiaomi-riva (replaced with generic package)

Supported devices with the generic device package:

msm8917 (upstreamed):

• Redmi 5A (xiaomi-riva) (upstreamed)
• Redmi Note 5A (xiaomi-ugglite)

msm8937:

• Redmi 3S (xiaomi-land) (custom lk2nd build is needed)

msm8940:

• Redmi 4X (xiaomi-santoni)
• Redmi Note 5A Prime (xiaomi-ugg)

TODO:

• make a stable branch at the kernel fork (msm8917 upstreaming is in progress)
• clarify the maintainers (are the old maintainers still active, is there anyone who want to be a maintainer)

Fixes: #3299 (closed)

!5603 (closed)