pine64-rockpro64: use alpine's u-boot-rockchip

Drop u-boot-rockpro64 in favor of using the u-boot package from Alpine.

Do this because u-boot-rockpro64 doesn't build anymore, and according to the comment in the APKBUILD it should now be possible to use u-boot from Alpine anyway.

@PureTryOut @MartijnBraam: I don't have a RP64, can you build an sdcard with this and test if it boots up? it was confirmed below that it boots.