powerpc: enable building all dtbs

Enable the 'dtbs' target for powerpc. This allows building all the dts
files in arch/powerpc/boot/dts/ when COMPILE_TEST and OF_ALL_DTBS are
enabled.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Rob Herring <robh@kernel.org>

arch/powerpc/boot/dts/Makefile

 # SPDX-License-Identifier: GPL-2.0
+
+subdir-y += fsl
+
+dtstree		:= $(srctree)/$(src)
+dtb-$(CONFIG_OF_ALL_DTBS) := $(patsubst $(dtstree)/%.dts,%.dtb, $(wildcard $(dtstree)/*.dts))

arch/powerpc/boot/dts/fsl/Makefile

+# SPDX-License-Identifier: GPL-2.0
+
+dtstree		:= $(srctree)/$(src)
+dtb-$(CONFIG_OF_ALL_DTBS) := $(patsubst $(dtstree)/%.dts,%.dtb, $(wildcard $(dtstree)/*.dts))