powerpc: use kbuild generic-y where possible

Replace one line asm-generic include files declared in
arch/powerpc/include/asm/ by generic-y declaration
which creates arch/powerpc/include/generated/asm equivalent.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>