powerpc/8xx: Fix two CONFIG_8xx left behind

Commit 968159c0 ("powerpc/8xx: Getting rid of remaining use of
CONFIG_8xx") removed all but 2 references to 8xx in Kconfigs.

This patch removes the two remaining ones.

Fixes: 968159c0 ("powerpc/8xx: Getting rid of remaining use of CONFIG_8xx")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>