powerpc/dts: Build virtex dtbs

I wanted to test the virtex440-ml507 qemu machine and found that the
dtb for it was not built.

All powerpc dtbs are only built when CONFIG_OF_ALL_DTBS is set which
depend on COMPILE_TEST.

This patch enables building of the virtex dtbs when
CONFIG_XILINX_VIRTEX440_GENERIC_BOARD is enabled.
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
[mpe: Put both targets on a single line]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/boot/dts/Makefile

@@ -4,3 +4,4 @@ subdir-y += fsl
 
 dtstree		:= $(srctree)/$(src)
 dtb-$(CONFIG_OF_ALL_DTBS) := $(patsubst $(dtstree)/%.dts,%.dtb, $(wildcard $(dtstree)/*.dts))
+dtb-$(CONFIG_XILINX_VIRTEX440_GENERIC_BOARD) += virtex440-ml507.dtb virtex440-ml510.dtb