memory: omap-gpmc: Fix build with CONFIG_OMAP_GPMC disabled

Fix the following build failure if omap-gpmc.h is
used with CONFIG_OMAP_GPMC disabled.

./include/linux/omap-gpmc.h:32:1: error: unknown type name ‘gpmc_nand_ops’

Signed-off-by: Roger Quadros <rogerq@ti.com>