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>