u-boot-rpi: Enable rpi4 target on armhf and arm64

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=958668

I've now personally tested the arm64 version on real hardware, using a patched build of u-boot 2020.04+dfsg-2 from unstable, verifying u-boot starts and I get a serial console hush shell. (If needed I could also test armhf and/or building the git version on request.)

I've listed both me and Lucas as contacts for the rpi_4 target.