powerpc/dts: Makefile changes to clean and build dts

Signed-off-by: Jagdish Gediya <jagdish.gediya@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>

arch/powerpc/dts/Makefile

+# SPDX-License-Identifier: GPL-2.0+
+
+targets += $(dtb-y)
+
+# Add any required device tree compiler flags here
+DTC_FLAGS +=
+
+PHONY += dtbs
+dtbs: $(addprefix $(obj)/, $(dtb-y))
+	@:
+
+clean-files := *.dtb

dts/Makefile

@@ -61,4 +61,4 @@ dtbs: $(obj)/dt.dtb $(obj)/dt-spl.dtb
 clean-files := dt.dtb.S dt-spl.dtb.S
 
 # Let clean descend into dts directories
-subdir- += ../arch/arm/dts ../arch/microblaze/dts ../arch/mips/dts ../arch/sandbox/dts ../arch/x86/dts
+subdir- += ../arch/arm/dts ../arch/microblaze/dts ../arch/mips/dts ../arch/sandbox/dts ../arch/x86/dts ../arch/powerpc/dts