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>