OMAP devices support various NAND transfer modes.

Currently all device-tree definitions will use the default "prefetch
polled" mode, so this patch enables the transfer mode to be specified
in the device-tree.
Signed-off-by: Mark Jackson <mpfj@newflow.co.uk>
Acked-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>