mtd: nand: pxa3xx: Add support for 2048 bytes page size devices

This commit adds support for devices with 2048B page sizes and
4-bit ECC strength requirements. This is achieved by enabling the BCH
ECC engine, which provides a higher strength: 16-bit over 2048 bytes.

Additionally, add a proper ECC layout to model the controller's view
of the device (where 'U' means unused and 'B' is the bad block marker):

 ----------------------------------------------------
 | 2048B data | B | B | 30B spare | 30B ECC | U | U |
 ----------------------------------------------------

Signed-off-by: Rodolfo Giometti <giometti@linux.it>
[Brian: updated with Ezequiel's patch description]
Acked-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>