nand: extend .raw accesses to work on multiple pages

nand: extend .raw accesses to work on multiple pages

A use for this is to read, modify, erase, and write an entire block as a
single unit, as a replacement for the biterr command.  This way gives
more flexibility in that you can also test multiple bit errors, errors
in the ECC, etc.

Signed-off-by: Scott Wood <scottwood@freescale.com>