xiaomi-santoni: moved to a new branch - use msm-3.18 upstream kernel

As requested by @ollieparanoid , I created a new branch and a new pull request due to myself messed up. Here's the old commit message:

The current source being used for Xiaomi Redmi 4X (santoni) postmarketOS is from Xiaomi repo, which is pretty old 3.18.3x, and since we don't want outdated stuffs, I upstreamed the kernel with 3.18.136, which is from: https://github.com/android-linux-stable/msm-3.18

The kernel compiles, but does not boot with GCC 8, so GCC 6 is still being used.

I have tested the kernel, USB Networking, Display, USB OTG, Wi-Fi (with proprietary blobs) works fine, through I haven't figured out how to get Bluetooth and among of other stuffs to work.

EDIT: Audio works. For more information on how to get Wi-Fi and Audio to work, check out the wiki page for this device, I have updated it. https://wiki.postmarketos.org/wiki/Xiaomi_RedMi4x_(xiaomi-santoni)