Draft: pine64-pinetab-v: new device
This PR tries to add Pine64 PineTab-V support to postmarketOS.
Image building currently have some problem -- the bootloader in SPI flash expects the boot partition to be the 3rd partition instead of the 1st, so currently partitioning should be manual, and split images could be flashed to the boot partition as the 3rd partition of SD card (or maybe eMMC, untested because other OS is on it) and the root partition as any other partition number.