powerpc/setup: replace ifdefs by IS_ENABLED() wherever possible.

Compared to ifdefs, IS_ENABLED() provide a cleaner code and allows
to detect compilation failure regardless of the selected options.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>