mtd: OMAP: Enable GPMC prefetch mode for 16 bit access

commit c316f577 "mtd: OMAP: Enable GPMC prefetch mode" only enabled
prefetch mode for 8 bit nand access, this adds 16 bit as well.

Cc: Scott Wood <scottwood@freescale.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Daniel Mack <zonque@gmail.com>

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Reviewed-by: Tom Rini <trini@konsulko.com>