mtd: atmel_nand: don't map the ROM table if no pmecc table offset in DT

if atmel,pmecc-lookup-table-offset is not found in DT node, we don't
need to map the ROM table as we will build a runtime gf table anyway.

Reported-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>