mtd: support writing OOB without ECC

This fixes issues with `nandwrite -n -o' and the MEMWRITEOOB[64] ioctls
on hardware that writes ECC when writing OOB. The problem arises as
follows: `nandwrite -n' can write page data to flash without applying
ECC, but when used with the `-o' option, ECC is applied (incorrectly),
contrary to the `--noecc' option.

I found that this is the case because my hardware computes and writes
ECC data to flash upon either OOB write or page write. Thus, to support
a proper "no ECC" write, my driver must know when we're performing a raw
OOB write vs. a normal ECC OOB write. However, MTD does not pass any raw
mode information to the write_oob functions.  This patch addresses the
problems by:

1) Passing MTD_OOB_RAW down to lower layers, instead of just defaulting
   to MTD_OOB_PLACE
2) Handling MTD_OOB_RAW within the NAND layer's `nand_do_write_oob'
3) Adding a new (replaceable) function pointer in struct ecc_ctrl; this
   function should support writing OOB without ECC data. Current
   hardware often can use the same OOB write function when writing
   either with or without ECC

This was tested with nandsim as well as on actual SLC NAND.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Jim Quinlan <jim2101024@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>