powerpc: conditionally compile platform-specific serial drivers

mpsc.c and mpc52xx-psc.c are platform-specific serial drivers, and
should be compiled for the respective platforms only.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>