xiaomi-santoni: rebase to 3.18.138, add firmware and audio, fix splash (!328)

device-xiaomi-santoni:
* Added initial audio support (generated by xml2ucm)
* Fixed splash screen size (720x1280), which also fixes the screen
  showing "posma"

firmware-xiaomi-santoni:
* Firmware files are added (contains ADSP and WCNSS firmwares), these
* files are from Developer ROM 03/29 by Xiaomi.

linux-xiaomi-santoni:
* Kernel updated to 3.18.138.
* Modernized GCC, it'll now compile with the latest GCC from Alpine.

[ci:skip-build]: already built successfully in CI