cmd: mvebu: bubt: add A38x support

Add support for Armada 38x devices in bubt flashing utility.
This is based on (and streamlined from) the support in the SolidRun
master-a38x vendor fork.

Signed-off-by: Joel Johnson <mrjoel@lixil.net>
Reviewed-by: Stefan Roese <sr@denx.de>