u-boot-firefly-rk3399: Add a u-boot port for this board

This is a ARM64 board with a RockChip RK3399 SoC.

Add support for RK3399 to the u-boot-master port. No METADATA are created
for now as this u-boot is split in two file and we don't support this scheme
yet.
Two patch are needed :

• One to allow booting u-boot proper from the same source as the DTS (accepted and merge upstream)
• One to have the BL31 (ATF binary) path in a environment variable instead of in the source directory (accepted upstream).