mtd: nand: omap: Documentation: How to select correct ECC scheme for your device ?

- Adds DT binding property for BCH16 ECC scheme
 - Adds describes on factors which determine choice of ECC scheme for particular device

CC: devicetree@vger.kernel.org
Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>