mtd: extend plat_nand for (read|write)_buf

This patch adds (write|read)_buf callbacks to plat_nand.

The NAND on the TS-7800 provisioned by the FPGA allows readw() and
readl() to be used which gives a 2.5x speed up.  To be able to use this
from the plat_nand driver a hook for read_buf (and also write_buf whilst
we are in there) need to be made available.  This patch adds the hook.

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>