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

ELM hardware engine is used to detect ECC errors for BCHx ecc-schemes
(like BCH4/BCH8/BCH16). This patch extends configuration of ELM registers
for adding support of BCH16_HW ecc-scheme.

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