xiaomi-lavender: new device (Xiaomi Redmi Note 7)

This adds basic support for Xiaomi Redmi Note 7 and finishes some work that is stuck as WIP for too long already.

2 kernels variants are supported: downstream and mainline.

For downstream kernel:

Works:

• flashing with fastboot, including vbmeta
• kernel boots
• ssh over USB
• framebuffer (pmOS splash screens, wayland-based UIs)
• weston works, and Xwayland is running
• touchscreen
• battery is charging

Does not work:

• Wi-Fi is blocked by #494 (closed) and unlikely to work any time soon
• X11-based UIs crash: Xorg log

Mainline kernel it only at embryo stage. The main problem is that there is no sdm660 SoC in mainline kernel currently, only some individual drivers that were contributed by @opendata26. This continues opendata's work and serves as proof of concept that this is possible.

Works:

• kernel boots
• dmesg logs in pstore fs (you can read them from TWRP recovery)
• dmesg logs over UART interface
• all 8 CPUs are up!
• simple-framebuffer can show logs of booting kernel on the screen because it uses hw initialized by bootloader
• booting process successfully reaches postmarketOS's initramfs and it (obviously) can't find partitions because no block devices are present at all

Doesn't work:

• everything else: no voltage regulators (PMIC) support => no USB, no sdhci, no gpu, ... no nothing

P.S. the following features were originally implemented to support this device:

• make USB configfs attrs customizable !706 (merged)
• pmbootstrap: add "flasher flash_vbmeta" command pmbootstrap!1885 (merged)

P.P.S. funny note: nixOS-mobile lavender MR that uses patches from my branch was merged almost 4 months ago... so I took one patch from samuel too :)