mtd: nand: omap: add support for BCH16_ECC - NAND driver updates

This patch add support for BCH16 ecc-scheme in OMAP NAND driver, by extending
following functions:
 - omap_enable_hwecc (nand_chip->ecc.hwctl): configure GPMC controller
 - omap_calculate_ecc_bch (nand_chip->ecc.calculate): fetch ECC signature from GPMC controller
 - omap_elm_correct_data (nand_chip->ecc.correct): detect and correct ECC errors using ELM

(a) BCH16 ecc-scheme can detect and correct 16 bit-flips per 512Bytes of data.
(b) BCH16 ecc-scheme generates 26-bytes of ECC syndrome / 512B.
Due to (b) this scheme can only be used with NAND devices which have enough
OOB to satisfy the relation: "OOBsize per page >= 26 * (page-size / 512)"

Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>