cmd: mvebu/bubt: Add support for sha512 checksum validation for Armada 3700

Armada 3700 BootROM supports also images with sha512 checksums and
mox-imager tool [1] generates such bootable images. Without sha512 support
U-Boot bubt command just prints error message:

  Error: Unsupported hash_algorithm_id = 64
  Error: Image header verification failed

This patch adds support for sha512 checksum validation for Armada 3700
images. With it bubt prints:

  Image checksum...OK!

[1] - https://gitlab.nic.cz/turris/mox-boot-builder.git



Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>