mtd: nand: denali: fix raw and oob accessors for syndrome page layout

The Denali IP adopts the syndrome page layout; payload and ECC are
interleaved, with BBM area always placed at the beginning of OOB.

The figure below shows the page organization for ecc->steps == 2:

  |----------------|    |-----------|
  |                |    |           |
  |                |    |           |
  |    Payload0    |    |           |
  |                |    |           |
  |                |    |           |
  |                |    |           |
  |----------------|    |  in-band  |
  |      ECC0      |    |   area    |
  |----------------|    |           |
  |                |    |           |
  |                |    |           |
  |    Payload1    |    |           |
  |                |    |           |
  |                |    |           |
  |----------------|    |-----------|
  |      BBM       |    |           |
  |----------------|    |           |
  |Payload1 (cont.)|    |           |
  |----------------|    |out-of-band|
  |      ECC1      |    |    area   |
  |----------------|    |           |
  |    OOB free    |    |           |
  |----------------|    |-----------|

The current raw / oob accessors do not take that into consideration,
so in-band and out-of-band data are transferred as stored in the
device.  In the case above,

  in-band:      Payload0 + ECC0 + Payload1(partial)
  out-of-band:  BBM + Payload1(cont.) + ECC1 + OOB-free

This is wrong.  As the comment block of struct nand_ecc_ctrl says,
driver callbacks must hide the specific layout used by the hardware
and always return contiguous in-band and out-of-band data.

The current implementation is completely screwed-up, so read/write
callbacks must be re-worked.

Also, it is reasonable to support PIO transfer in case DMA may not
work for some reasons.  Actually, the Data DMA may not be equipped
depending on the configuration of the RTL.  This can be checked by
reading the bit 4 of the FEATURES register.  Even if the controller
has the DMA support, dma_set_mask() and dma_map_single() could fail.
In either case, the driver can fall back to the PIO transfer.  Slower
access would be better than giving up.

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