mtd: atmel_nand: make PMECC lookup table and offset property optional

If there is no PMECC lookup table stored in ROM, or lookup table offset is
not specified, PMECC driver should build it in DDR by itself.

That make the PMECC driver work for some board which doesn't have PMECC
lookup table in ROM.

The PMECC use the BCH algorithm, so based on the build_gf_tables()
function in lib/bch.c, we can build the Galois Field lookup table.

For more information can refer to section 5.4 of PMECC controller
application note:
	http://www.atmel.com/images/doc11127.pdf



Signed-off-by: Josh Wu <josh.wu@atmel.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Brian Norris <computersforpeace@gmail.com>