powerpc/32: don't use CPU_FTR_COHERENT_ICACHE

Only 601 and E200 have CPU_FTR_COHERENT_ICACHE.

Just use #ifdefs instead of feature fixup.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5f3e92ccd64d06477b27626f6007a9da3b8da157.1566834712.git.christophe.leroy@c-s.fr