powerpc: Remove unused config in the Makefile

The patch below removes an unused config variable found by using a kernel
cleanup script.
Note: I did try to cross compile these but hit erros while doing so..
(gcc is not setup to cross compile) and am unsure if anymore needs to be done.
Please have a look if/when anybody has free time.

Signed-off-by: Justin P. Mattock <justinmattock@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>