powerpc: Enable kernel XZ compression option on 44x

Enable kernel XZ compression option on 44x.
Tested on a Western Digital - MyBook Live NAS.
It takes 22 seconds for the 800 MHz CPU to decompress
and boot a 2.63 MiB XZ-compressed kernel simpleImage.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>