mtd: nand: denali: skip driver internal bounce buffer when possible

For ecc->read_page() and ecc->write_page(), it is possible to call
dma_map_single() against the given buffer.  This bypasses the driver
internal bounce buffer and save the memcpy().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>