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 <masahiroy@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>