mtd: at91: add dt parameters for Atmel PMECC

Add DT support for PMECC parameters.

Signed-off-by: Hong Xu <hong.xu@atmel.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>