omap3: nand: configurable transfer type per board

nand transfer type (sDMA, Polled, prefetch) can be select from board file,
enabling all transfer type in driver, by default.

this helps in multi-omap build and to select different transfer type for
different board.

Signed-off-by: Sukumar Ghorai <s-ghorai@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>