mtd: nand: omap_elm: use macros for register definitions

This patch adds macros for following parameters of ELM Hardware engine
 - ELM_MAX_CHANNELS: ELM can process 8 data streams simultaneously
 - ELM_MAX_ERRORS: ELM can detect upto 16 ECC error when using BCH16 scheme

Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Stefan Roese <sr@denx.de>