Use "rpi_arm64" config in U-Boot

As of the v2020.01 release of U-Boot there is now a "rpi_arm64" configuration that will build a U-Boot that works on any Raspberry Pi board in arm64 (rather than arm32) mode.  Long term the intention is that any new Pi's will work with this config as well.  It is likely that long term there will also be a single MACHINE that will work on all 64bit Raspberry Pi platforms.  We should keep this in mind when moving to "Dunfell" or later code-name release of OpenEmbedded (as that is the minimum release that will contain v2020.01 or later).