boards: tam3517-common: enable gpmc prefetch mode

Since the tam3517 base board has a 16bit wide nand connected to
the gpmc, enable the prefetch mode, since that is now supported.

Cc: Scott Wood <scottwood@freescale.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: pekon gupta <pekon@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Daniel Mack <zonque@gmail.com>
Cc: Tapani Utriainen <tapani@technexion.com>

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