powerpc: Remove unnecessary casts of void ptr

Hi,

The [vk][cmz]alloc(_node) family of functions return void pointers which
it's completely unnecessary/pointless to cast to other pointer types since
that happens implicitly.

This patch removes such casts from arch/powerpc/

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>