[POWERPC] Fix build of cell zImage.initrd

The patch adding support for zImage.ps3 didn't add a zImage.initrd.ps3
target causing builds of zImage.initrd to fail when ps3 is included in
the .config. The current method of generating ps3 images doesn't support
initrd's yet, so we create a dummy target that only displays a warning
message instead.

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