libfdt: overlay_merge: Ignore unresolved symbols

fdt_overlay_merge(), like fdt_overlay_apply(), has to perform similar operations
on DT blobs, one of which is going through external symbol references specified
in __fixups__ node of overlay DT blob and resolving them in base DT blob (as
performed by overlay_fixup_phandles()).

Unlike overlay case though, in case of merging two overlay blobs, its quite
normal that some of the external references specified in __fixups__ node are not
found in base blob. Modify overlay_fixup_phandles() to understand this
possibility.

Change-Id: I1375e2d1d074af6b11c0c86fdd55a5bf45b3fb10
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>