oneplus-kebab: new port

I successfully ported pmOS to the OnePlus 8T. I've already created a wiki page at https://wiki.postmarketos.org/wiki/OnePlus_8T_(oneplus-kebab), though it's incomplete. I'll update it ASAP.

USB Networking, Display with DRM and Weston and the Touchscreen are working.

When compiling the kernel with CONFIG_BUILD_ARM64_DT_OVERLAY enabled, the dtbo partition has to contain the same dtbo-s. I don't know if there is any best practice about how and when to generate a dtbo.img, so I hope that my soloution is acceptable. Without device tree overlays the touchscreen device does not work properly.