mtd: atmel_nand: add has_pmecc member

This commit replace the usage of CONFIG_NAND_ATMEL_PMECC
by a new member called: has_pmecc. This member is directly initialise
in board specific code.

Signed-off-by: Raphaël Poggi <poggi.raph@gmail.com>
arm: board: initialise member has_pmecc

Initialise the member has_pmecc on boards which support it.

Signed-off-by: Raphaël Poggi <poggi.raph@gmail.com>
mtd: atmel_nand: add SOFT_BCH support

To use soft ecc with nand with 8k page, we need the BCH support.
This commit adds the possibilty for atmel_nand to enable SOFT_BCH.

Signed-off-by: Raphaël Poggi <poggi.raph@gmail.com>