mtd: nand: pxa3xx: simplify ECC hardware parameters

Replace the if/else chain in pxa_ecc_init() with a lookup table. This
makes the code more concise and hopefully easier to follow. Remove the
unused ecc_layout tables and replace it with a single dummy one (the
pxa3xx driver has never used this but the mtd subsystem expects it to be
provided).

Tested on an Allied Telesis x530 switch with Micron MT29F2G08ABAEAWP
NAND Flash.

Signed-off-by: Chris Packham <judge.packham@gmail.com>