fdt: Add correct return for early_init_dt_scan_memory

For currently if return 0 it will search the whole
device tree blob for all device type memory. It will
affect the unnecessary boot up time.

Change-Id: I688b54eda224b744bbc7e706f8b70cccdf8cc38c
Signed-off-by: Maria Yu <aiquny@codeaurora.org>