sunxi: arm64: Add addresses for compressed kernel load

The "booti" command to load arm64 Linux kernels supports automatic
decompression of zipped kernel images, but relies on some environment
variables to point to usable buffer RAM.

Add those variables and let them point to some default values, that
should cover most use-cases.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>