Draft: new device: xiaomi-latte(aka Mi Pad 2)

This PR is for adding new device Xiaomi Pad 2, codename "latte".

This is still working in progress, currently I can flush and boot, but stuck at the booting screen and only see the postmarketOS logo and the "loading..." text, I'm not familiar with debugging so I can't figure out how to debug the booting process, I tried adding the debug hooks into initramfs, but ssh and telnet both failed.

Latest progress:

Up to 11/15/2020, I can boot into system using "weston" UI without any further configuration(change "UI" in ~/.config/pmbootstrap.conf to "weston" then install and flush), but sadly touchscreen is not working, also ssh/telnet is still not working, so there is nothing to play with except long press power button to shutdown.