powerpc/boot: Fix build of dts/fsl

The lkp robot reported that some configs fail to build, for example
mpc85xx_smp_defconfig, with:

  cc1: fatal error: opening output file arch/powerpc/boot/dts/fsl/.mpc8540ads.dtb.dts.tmp: No such file or directory

This bisects to:
  cc8a51ca ("kbuild: always create directories of targets")

Although that commit claims to be about in-tree builds, it somehow
breaks out-of-tree builds. But presumably it's just exposing a latent
bug in our Makefiles.

We can fix it by adding to targets for dts/fsl in the same way that we
do for dts.

Fixes: cc8a51ca

 ("kbuild: always create directories of targets")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201215032906.473460-1-mpe@ellerman.id.au