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>