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: Raphal Poggi <poggi.raph@gmail.com>
arm: board: initialise member has_pmecc

Initialise the member has_pmecc on boards which support it.

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