release: Rename arm64/RPI3.conf to arm64/RPI.conf

We now have a u-boot port and config.txt for booting on all 64bits
variant of the RPI boards (RPI2v1.1, RPI3* and RPI4*) so use
the new u-boot-rpi-arm64 and the config_arm64.txt files.

Discussed with: karels, kevans