powerpc/cpm1: link to CONFIG_CPM1 instead of CONFIG_8xx

To remain consistent with what is done with CPM2, let's link
CPM1 related parts to CONFIG_CPM1 instead of CONFIG_8xx

When something depends on both CPM1 and CPM2 we associate it
with CONFIG_CPM

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