mtd: nand: omap: ecc.hwctl: clean omap_enable_hwecc_bch for redundant ECC configs

This patch
 - refactors GPMC configurations based on ecc-scheme
 - removed dependency on is_elm_present() flag, which is implicitely
   taken care by selecting appropriate ecc-scheme

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