[POWERPC] Remove 'make zImage.dts' feature

Being able to selectively wrap a device tree with the zIimage at build
time has been deemed unnecessary, so this removes Makefile support for
that feature.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>