powerpc: enable heap randomization for linkstations

The current defconfig for Linkstation/Kuroboxes has the "Disable Heap
Randomization" option enabled.

Since some of these machines are facing the internet, it helps to have
heap randomization enabled. This patch enables it.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>