powerpc: Add cuImage.mpc866ads to the bootwrapper as a cuboot-8xx target

This patch fixes the following build error with mpc866_ads_defconfig:

<--  snip  -->

...
  WRAP    arch/powerpc/boot/cuImage.mpc866ads
powerpc64-linux-ld: arch/powerpc/boot/cuboot-mpc866ads.o: No such file: No such file or directory
make[2]: *** [arch/powerpc/boot/cuImage.mpc866ads] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>