mtd: onenand: add new callback for bufferram read

This patch adds a new callback for the underlying drivers, which is
called instead of accessing the buffer ram directly. This callback will
be used by Samsung OneNAND driver to implement DMA transfers on S5PC110
SoC.

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>