powerpc/boot: add DTB to 'targets'

PowerPC always re-builds DTB even if nothing has been changed.

As for other architectures, arch/*/boot/dts/Makefile builds DTB by
using the dtb-y syntax.

In contrast, arch/powerpc/boot/dts/(fsl/)Makefile does nothing unless
CONFIG_OF_ALL_DTBS is defined. Instead, arch/powerpc/boot/Makefile
builds DTB on demand. You need to add DTB to 'targets' explicitly
so .*.cmd files are included.

Signed-off-by: Masahiro Yamada <[e-mail]>
Acked-by: Michael Ellerman <[e-mail]>

arch/powerpc/boot/Makefile

···
 		$(patsubst treeImage%, treeImage.initrd%, $(image-y)))))
 initrd-y := $(filter-out $(image-y), $(initrd-y))
 targets	+= $(image-y) $(initrd-y)
+targets += $(foreach x, dtbImage uImage cuImage simpleImage treeImage, \
+		$(patsubst $(x).%, dts/%.dtb, $(filter $(x).%, $(image-y))))
 
 $(addprefix $(obj)/, $(initrd-y)): $(obj)/ramdisk.image.gz