memory: fix build when CONFIG_OF && !CONFIG_DDR

Commit e6b42eb6 "memory: emif: add device tree support to emif driver"
added drivers/memory/of_memory.c, which references tables defined in
lib/jedec_ddr_data.c. of_memory.c is compiled when CONFIG_OF, whereas
jedec_ddr_data.c is compiled when CONFIG_DDR. This breaks the build
when CONFIG_OF is defined but not CONFIG_DDR:

drivers/built-in.o: In function `of_get_ddr_timings':
drivers/memory/of_memory.c:138: undefined reference to `lpddr2_jedec_timings'
drivers/built-in.o: In function `of_get_min_tck':
drivers/memory/of_memory.c:62: undefined reference to `lpddr2_jedec_min_tck'
make: *** [vmlinux] Error 1

To solve this, only compile of_memory.c when CONFIG_OF && CONFIG_DDR,
otherwise, stub out the functions.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/memory/Makefile

@@ -2,7 +2,9 @@
 # Makefile for memory devices
 #
 
+ifeq ($(CONFIG_DDR),y)
 obj-$(CONFIG_OF)		+= of_memory.o
+endif
 obj-$(CONFIG_TI_EMIF)		+= emif.o
 obj-$(CONFIG_TEGRA20_MC)	+= tegra20-mc.o
 obj-$(CONFIG_TEGRA30_MC)	+= tegra30-mc.o

drivers/memory/of_memory.h

@@ -12,7 +12,7 @@
 #ifndef __LINUX_MEMORY_OF_REG_H
 #define __LINUX_MEMORY_OF_REG_H
 
-#ifdef CONFIG_OF
+#if defined(CONFIG_OF) && defined(CONFIG_DDR)
 extern const struct lpddr2_min_tck *of_get_min_tck(struct device_node *np,
 		struct device *dev);
 extern const struct lpddr2_timings
@@ -31,6 +31,6 @@ static inline const struct lpddr2_timings
 {
 	return NULL;
 }
-#endif /* CONFIG_OF */
+#endif /* CONFIG_OF && CONFIG_DDR */
 
 #endif /* __LINUX_MEMORY_OF_REG_ */