boards: Add rockpi4 documentation

Signed-off-by: Nícolas F. R. A. Prado nfraprado@collabora.com

Note: I got the load addresses from Radxa's PXE setup: https://wiki.radxa.com/Rockpi4/dev/u-boot/pxe . u-boot relocates the kernel from 0x280000 to 0x400000, but everything works well nonetheless.