powerpc/8xxx: Fix possible compile issue related to P1013

The P1013 is a single core version of P1022 and thus should use the
p1022_serdes.c code.  It was acciently pointing to p1013_serdes.c which
doesn't exist.

Reported-by: Renaud Barbier <renaud.barbier@ge.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>