mtd: nand: Added a device flag for subpage read support

Added a NAND device flag for subpage read support. Previously this was
hard coded based on large page and soft ECC.
Updated base NAND driver to use the new subpage read flag if the NAND is
large page and soft ECC.

Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
Reviewed-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>