[POWERPC] Fix zImage decompress location

The zImage wrapper has a "hack" that force the decompression to happen
above 20Mb for 64 bits kernels, to work around issues with some
firmwares on the field. However, the new wrapper has a bug which makes
that hack not work properly. This fixes it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>