mtd: onenand: omap2: Unify OMAP2 and OMAP3 DMA implementation

Since the very first commit (36cd4fb5

: "[MTD] [OneNAND] Add
OMAP2 / OMAP3 OneNAND driver") DMA is disabled for OMAP2. Later
fixes thus went only into OMAP3 specific DMA functions which
turned out not to be so OMAP3 specific, so merge those two
implementations.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Roger Quadros <rogerq@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>