[POWERPC] Remove CPU_FTR_NEED_COHERENT for 7448.

Remove CPU_FTR_NEED_COHERENT for MPC7448 (and single-core MPC86xx).
This prevents needlessly setting M=1 when not SMP.

Signed-off-by: James.Yang <James.Yang@freescale.com>
Acked-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>