xiaomi-clover: new device (Xiaomi Mi Pad 4)

This is a mainline kernel fork port for a Xiaomi Mi Pad 4 device.

Works:

• USB;
• simple-framebuffer;
• touchscreen;
• buttons. Everything else does not currently work.

I hosted kernel source on my Github repository, but I'm not sure if that's the best way to do that. Is there a better way though?