mtd: nand: omap2: handle nand on gpmc

GPMC platform initialization has been modified to fill NAND
platform data with GPMC NAND register details. As these
registers are accessible in NAND driver itself, configure
NAND in GPMC by itself.

Modified prefetch and ecc functions are logically same as
the corresponding exported symbols from GPMC code.

Note: Verfying that other CS have not yet enabled for
prefetch & ecc has to be incorporated. Currently this
causes no issues as there are no boards that use NAND
on multiple CS. With ongoing GPMC driver migration,
perhaps it would be better to consider NAND connected
on multiple CS as a single peripheral using multiple CS.
This would make handling multiple CS issues easier.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>