of: only scan for reserved mem when fdt present

When the reserved memory patches hit -next, several legacy (non-DT) boot
failures were detected and bisected down to that commit. There needs to
be some sanity checking whether a DT is even present before parsing the
reserved ranges.

Reported-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>