Adds a generic device target for the Rockchip RK3228 and RK3229 SoCs with mainline U-Boot and kernel. I tested this on my MXQ Pro 4K STB, which originally had Android 7.1 installed. I've been testing it on a 32 GB SD card, as I currently have [ilmich's LibreELEC port](https://github.com/rk-unofficial-cfw/LibreELEC.tv/tree/le12-rk322x-wip] installed to the eMMC, though it should boot from eMMC just fine. The bulk of the patches comes from the aforementioned LibreELEC port and Armbian. Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/6982>