powerpc/5200: Enable CPU_FTR_NEED_COHERENT for MPC52xx

BestComm, a DMA engine in MPC52xx SoC, requires snooping when
CPU caches are enabled to work properly.

Adding CPU_FTR_NEED_COHERENT fixes NFS problems on MPC52xx machines
introduced by 'powerpc/mm: Fix handling of _PAGE_COHERENT in BAT setup
code' (sha1: 4c456a67).

Signed-off-by: Piotr Ziecik <kosmo@semihalf.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>