[POWERPC] Update holly to use new dts wrapping feature

The holly support currently has separate rules to wrap its device tree
with its zImage.  This can now be done automatically without the extra
rules so update holly support to use the automatic feature.

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