mtd: nand: pxa3xx: Add multiple chunk write support

This commit adds write support for large pages (4 KiB, 8 KiB).
Such support is implemented by issuing a multiple command sequence,
transfering a set of 2 KiB chunks per transaction.

The splitted command sequence requires to send the SEQIN command
independently of the PAGEPROG command and therefore it's set as
an execution command.

Since PAGEPROG enables ECC, each 2 KiB chunk of data is written
together with ECC code at a controller-fixed location within
the flash page.

Currently, only devices with a 4 KiB page size has been tested.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>