configs/rockpro64: Use mainline ATF

Use the mainline ATF as it supports the Pine64 ROCKPro64 board.

Fixes:
- https://gitlab.com/buildroot.org/buildroot/-/jobs/2812054016



Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>