u-boot-rpi: Add rpi_arm64 target for arm64

This target allow to use the same 64b U-Boot binary for both Raspberry Pi 3 and 4 versions. Tested the boot with both boards.

This target is used in Apertis (Debian's derivative) to produce a single bootable image for Raspberry Pi 3 and 4 with U-Boot.

Closes: #966078

Signed-off-by: Denis Pynkin denis.pynkin@collabora.com