nand: Try to align the default buffers

The NAND layer needs to use cache-aligned buffers by default. Towards this
goal. align the default buffers and their members according to the minimum
DMA alignment defined for the architecture.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>
Acked-by: Scott Wood <scottwood@freescale.com>