xiaomi-santoni: rebased to 3.18.138. firmware files, fixed splash screen and initial audio support

In this pull request, I've made some changes to this device, here are the changes:

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.