omap_gpmc: move prefetch out of CONFIG_NAND_OMAP_ELM

The prefech mode is a feature of the gpmc, not the ELM. An am3517
does not have an elm, but can do prefeches, so move the code out
of the CONFIG_NAND_OMAP_ELM ifdef.

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>