omap: fix the compile error if CONFIG_MTD_NAND_OMAP2 is notenabled

If CONFIG_MTD_NAND_OMAP2 is not enabled, there will be a compile error,
"gpmc_nand_init() is not defined". Add a inline noop function to fix it.
Signed-off-by: Stanley.Miao <stanley.miao@windriver.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/plat-omap/include/plat/nand.h

@@ -29,4 +29,11 @@ struct omap_nand_platform_data {
 /* size (4 KiB) for IO mapping */
 #define	NAND_IO_SIZE	SZ_4K
 
+#if defined(CONFIG_MTD_NAND_OMAP2) || defined(CONFIG_MTD_NAND_OMAP2_MODULE)
 extern int gpmc_nand_init(struct omap_nand_platform_data *d);
+#else
+static inline int gpmc_nand_init(struct omap_nand_platform_data *d)
+{
+	return 0;
+}
+#endif