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

This patch add support for BCH16_ECC in GPMC (controller) driver:
- extends configuration space to include BCH16 registers
- extends parsing of DT binding for selecting BCH16 ecc-scheme

Signed-off-by: Pekon Gupta <pekon@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>