powerpc/corenet_ds: add back buffer write for NOR flash

Enable buffer write for better performance. This platform uses a NOR flash
chip which supports write buffer programming. CFI driver can query the
buffer size and use it to program the flash for best performance.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>